In preparation for improving the `base58` error types crate an `error`
module and move the single current error type there. Make the module
public and reexport the type.
Add a new `base58` crate to the workspace and move the `bitcoin::base58`
module to it.
Done as part of crate smashing, specifically so that we can make `bip32`
into a separate crate.
Tobin C. Harding
Liam Aharon