rust: allow restricting get_bip39_wordlist to subset

This will allow to set the autocomplete keyboard to a subset of the
BIP39 wordlist to allow entering the last word of a 12/18 word
mnemonic using the keyboard, similar to how the 24th word can be
entered using a menu.

The keyboard is needed for 12/18 words as there are too many
candidates to show in a simple menu.

src/rust/bitbox02-rust/src/workflow/mnemonic.rs

@@ -177,7 +177,7 @@ pub async fn get() -> Result<zeroize::Zeroizing<String>, CancelError> {
     status(&format!("Enter {} words", num_words), true).await;
 
     // Provide all bip39 words to restrict the keyboard entry.
-    let bip39_wordlist = bitbox02::keystore::get_bip39_wordlist().unwrap();
+    let bip39_wordlist = bitbox02::keystore::get_bip39_wordlist(None);
 
     let mut word_idx: usize = 0;
     let mut entered_words = vec![zeroize::Zeroizing::new(String::new()); num_words];

src/rust/bitbox02/src/keystore.rs

@@ -174,16 +174,25 @@ impl Drop for Bip39Wordlist {
     }
 }
 
-pub fn get_bip39_wordlist() -> Result<Bip39Wordlist, ()> {
-    let mut result = Bip39Wordlist(vec![core::ptr::null(); BIP39_WORDLIST_LEN as usize]);
-    for i in 0..BIP39_WORDLIST_LEN {
-        let mut word_ptr: *mut u8 = core::ptr::null_mut();
-        match unsafe { bitbox02_sys::keystore_get_bip39_word(i, &mut word_ptr) } {
-            false => return Err(()),
-            true => result.0[i as usize] = word_ptr,
-        }
-    }
-    Ok(result)
+/// If indices is None, all BIP39 English words are returned, otherwise only the words of the given
+/// indices in the BIP39 English wordlist.
+pub fn get_bip39_wordlist(indices: Option<&[u16]>) -> Bip39Wordlist {
+    let indices = match indices {
+        Some(indices) => indices.to_vec(),
+        None => (0..BIP39_WORDLIST_LEN).collect(),
+    };
+    Bip39Wordlist(
+        indices
+            .into_iter()
+            .map(|i| {
+                let mut word_ptr: *mut u8 = core::ptr::null_mut();
+                match unsafe { bitbox02_sys::keystore_get_bip39_word(i, &mut word_ptr) } {
+                    false => panic!("get_bip39_wordlist"),
+                    true => word_ptr as _,
+                }
+            })
+            .collect(),
+    )
 }
 
 pub fn secp256k1_pubkey_compressed_to_uncompressed(