Currently rust-bip39 only supports bip39 word counts divisible by 6

[notmandatory]

Describe the bug

This issue is a reminder so we don't forget to fix this.

The WordCount enum allows users to pick word counts that aren't divisible by 6, but bdk is using the rust-bip39 crate which only allows word counts that are divisible by 6. See:

https://docs.rs/bip39/1.0.1/src/bip39/lib.rs.html#245-261

To Reproduce

Create a Mnemonic with WordCount of 15 words and an error is returned.

Expected behavior

When using the WordCount enum all the variants should be valid.

Build environment

• BDK tag/commit: v0.11.0
• OS+version: any
• Rust/Cargo version: 1.60
• Rust/Cargo target: any

Additional context

The next version of rust-bip39 should fix this issue with rust-bitcoin/rust-bip39#18.

This is the PR where bdk switched to using rust-bip39 bitcoindevkit/bdk#462

See also BIP-39.

[thunderbiscuit]

This is resolved by bitcoindevkit/bdk#875.