Add Keypair::try_from_base58_string

[juchiast]

Add a conversion method that doesn't panic on errors.

[joncinque]

Thanks for your contribution! While I think this makes sense, the Box<dyn Error> for the error type really shows the problem with errors in this crate.

We need to add a specific error type for Keypair and use that for this new function. It could probably look very similar to the base error returned by ed25519_dalek, which wraps a Box<dyn Error> if an std feature is enabled: https://docs.rs/signature/2.2.0/signature/struct.Error.html

Then in the future, we can remove ed25519_dalek types from the public interface of this crate, so we can finally upgrade the ed25519_dalek dependency.

All that to say: let's separately add the proper error type and std feature to this crate, then this PR can reuse that. What do you think?

[juchiast]

I think it make more sense to add a ParseKeypairError for these functions, rather than an opaque Error.