Add new mnemonic from entropy function

[poma-keeps-building]

Am I missing something or there is no way to create Mnemonic from existing Entropy? This PR adds a method to do so.

[poma-keeps-building]

are the build errors related to my commit? cargo clippy --workspace --all-targets -- -D warnings desn't complain on my machine. Nightly one does return the same error, but it also returns it on the current master branch as well. might be relevant

[prestwich]

no, the build errors are not related to your changes. CI is pretty out of date for this repo 😓