Remove `anyhow` and return ErrorKind errors

[notmandatory]

I am working on a cli bitcoin wallet project that is using tiny-bip39 and I've found that in order to handle error results from tiny-bip39 I need to map_err() and downcast() them from anyhow::Error.

My understanding is that anyhow isn't really meant to be used in libs which should return their own custom errors (possibly enhanced with thiserror as you do).

https://github.com/dtolnay/anyhow#comparison-to-thiserror

Would this project be open to removing anyhow and just returning your own custom error enum values in your function results ? If so I'm happy to work on a PR.