rust: move bitbox02::keystore::bip39_mnemonic_from_seed() to bip39 module

diff --git a/src/rust/bitbox02-rust/src/bip39.rs b/src/rust/bitbox02-rust/src/bip39.rs
index 5916c5e..ab952e8 100644
--- a/src/rust/bitbox02-rust/src/bip39.rs
+++ b/src/rust/bitbox02-rust/src/bip39.rs
@@ -26,6 +26,12 @@ pub fn get_word(idx: u16) -> Result<zeroize::Zeroizing<String>, ()> {
     ))
 }
 
+/// Encode a seed as a BIP39 mnemonic.
+pub fn mnemonic_from_seed(seed: &[u8]) -> Result<zeroize::Zeroizing<String>, ()> {
+    let mnemonic = bip39::Mnemonic::from_entropy(seed).map_err(|_| ())?;
+    Ok(zeroize::Zeroizing::new(mnemonic.to_string()))
+}
+
 /// Decode a BIP39 mnemonic.
 pub fn mnemonic_to_seed(mnemonic: &str) -> Result<zeroize::Zeroizing<Vec<u8>>, ()> {
     let mnemonic =
@@ -103,6 +109,33 @@ mod tests {
         assert_eq!(get_word(563).unwrap().as_ref() as &str, "edit");
     }
 
+    #[test]
+    fn test_mnemonic_from_seed() {
+        // 12 words
+        let seed = b"\xae\x6a\x40\x26\x1f\x0a\xcc\x16\x57\x04\x9c\xb2\x1a\xf5\xfb\xf7";
+        assert_eq!(
+            mnemonic_from_seed(seed).unwrap().as_str(),
+            "purpose faith another dignity proud arctic foster near rare stumble leave urge",
+        );
+
+        // 18 words
+        let seed = b"\x2a\x3e\x07\xa9\xe7\x5e\xd7\x3a\xa6\xb2\xe1\xaf\x90\x3d\x50\x17\xde\x80\x4f\xdf\x2b\x45\xc2\x4b";
+        assert_eq!(
+            mnemonic_from_seed(seed).unwrap().as_str(),
+            "clay usual tuna solid uniform outer onion found question limit favorite cook trend child lake hamster seat foot",
+        );
+
+        // 24 words
+        let seed = b"\x24\x1d\x5b\x78\x35\x90\xc2\x1f\x79\x69\x8e\x7c\xe8\x92\xdd\x03\xfb\x2c\x8f\xad\xc2\x44\x0e\xc2\x3a\xa5\xde\x9e\x2d\x23\x81\xb0";
+        assert_eq!(
+            mnemonic_from_seed(seed).unwrap().as_str(),
+            "catch turn task hen around autumn toss crack language duty resemble among ready elephant require embrace attract balcony practice rule tissue mushroom almost athlete",
+        );
+
+        // Invalid seed side
+        assert!(mnemonic_from_seed(b"foo").is_err());
+    }
+
     #[test]
     fn test_mnemonic_to_seed() {
         assert!(mnemonic_to_seed("invalid").is_err());
diff --git a/src/rust/bitbox02-rust/src/hww/api/show_mnemonic.rs b/src/rust/bitbox02-rust/src/hww/api/show_mnemonic.rs
index 4850730..b9654d9 100644
--- a/src/rust/bitbox02-rust/src/hww/api/show_mnemonic.rs
+++ b/src/rust/bitbox02-rust/src/hww/api/show_mnemonic.rs
@@ -34,7 +34,7 @@ pub async fn process(hal: &mut impl crate::hal::Hal) -> Result<Response, Error>
             crate::keystore::copy_seed()?
         };
 
-        bitbox02::keystore::bip39_mnemonic_from_seed(&seed)?
+        crate::bip39::mnemonic_from_seed(&seed)?
     };
 
     hal.ui()
diff --git a/src/rust/bitbox02-rust/src/keystore.rs b/src/rust/bitbox02-rust/src/keystore.rs
index ec992f0..078d3bd 100644
--- a/src/rust/bitbox02-rust/src/keystore.rs
+++ b/src/rust/bitbox02-rust/src/keystore.rs
@@ -84,7 +84,7 @@ pub fn create_and_store_seed(password: &str, host_entropy: &[u8]) -> Result<(),
 
 /// Returns the keystore's seed encoded as a BIP-39 mnemonic.
 pub fn get_bip39_mnemonic() -> Result<zeroize::Zeroizing<String>, ()> {
-    keystore::bip39_mnemonic_from_seed(&copy_seed()?)
+    crate::bip39::mnemonic_from_seed(&copy_seed()?)
 }
 
 fn get_xprv(keypath: &[u32]) -> Result<bip32::Xprv, ()> {
@@ -276,7 +276,7 @@ pub fn bip85_bip39(words: u32, index: u32) -> Result<zeroize::Zeroizing<String>,
     ];
 
     let entropy = bip85_entropy(&keypath)?;
-    keystore::bip39_mnemonic_from_seed(&entropy[..seed_size])
+    crate::bip39::mnemonic_from_seed(&entropy[..seed_size])
 }
 
 /// Computes a 16 byte deterministic seed specifically for Lightning hot wallets according to BIP-85.
diff --git a/src/rust/bitbox02/src/keystore.rs b/src/rust/bitbox02/src/keystore.rs
index 98ada42..5064586 100644
--- a/src/rust/bitbox02/src/keystore.rs
+++ b/src/rust/bitbox02/src/keystore.rs
@@ -14,7 +14,6 @@
 
 extern crate alloc;
 
-use alloc::string::{String, ToString};
 use alloc::vec;
 use alloc::vec::Vec;
 
@@ -231,11 +230,6 @@ pub fn _copy_bip39_seed() -> Result<zeroize::Zeroizing<Vec<u8>>, ()> {
     }
 }
 
-pub fn bip39_mnemonic_from_seed(seed: &[u8]) -> Result<zeroize::Zeroizing<String>, ()> {
-    let mnemonic = bip39::Mnemonic::from_entropy(seed).map_err(|_| ())?;
-    Ok(zeroize::Zeroizing::new(mnemonic.to_string()))
-}
-
 pub struct SignResult {
     pub signature: [u8; 64],
     pub recid: u8,
@@ -315,36 +309,8 @@ pub fn mock_unlocked(seed: &[u8]) {
 mod tests {
     use super::*;
     use bitcoin::secp256k1;
-
     use util::bb02_async::block_on;
 
-    #[test]
-    fn test_bip39_mnemonic_from_seed() {
-        // 12 words
-        let seed = b"\xae\x6a\x40\x26\x1f\x0a\xcc\x16\x57\x04\x9c\xb2\x1a\xf5\xfb\xf7";
-        assert_eq!(
-            bip39_mnemonic_from_seed(seed).unwrap().as_str(),
-            "purpose faith another dignity proud arctic foster near rare stumble leave urge",
-        );
-
-        // 18 words
-        let seed = b"\x2a\x3e\x07\xa9\xe7\x5e\xd7\x3a\xa6\xb2\xe1\xaf\x90\x3d\x50\x17\xde\x80\x4f\xdf\x2b\x45\xc2\x4b";
-        assert_eq!(
-            bip39_mnemonic_from_seed(seed).unwrap().as_str(),
-            "clay usual tuna solid uniform outer onion found question limit favorite cook trend child lake hamster seat foot",
-        );
-
-        // 24 words
-        let seed = b"\x24\x1d\x5b\x78\x35\x90\xc2\x1f\x79\x69\x8e\x7c\xe8\x92\xdd\x03\xfb\x2c\x8f\xad\xc2\x44\x0e\xc2\x3a\xa5\xde\x9e\x2d\x23\x81\xb0";
-        assert_eq!(
-            bip39_mnemonic_from_seed(seed).unwrap().as_str(),
-            "catch turn task hen around autumn toss crack language duty resemble among ready elephant require embrace attract balcony practice rule tissue mushroom almost athlete",
-        );
-
-        // Invalid seed side
-        assert!(bip39_mnemonic_from_seed(b"foo").is_err());
-    }
-
     #[test]
     fn test_derive_bip39_seed() {
         struct Test {