Repackaged base58 and removed _test

Moved base58 into its own sub-package for easy importing into other tools.

Addresses #7 for you.

This comment applies to all `Tst<Foo>` functions, but if we were to move away from `<pkg>_test` for the test files, there is no longer any need for `TstAddressPubKeyHash` since you can simply directly access the unexported fields of the `AddressPubKeyHash` struct.

These `Tst<Foo>` functions are provided via the `internal_test.go` as a bridge to expose some extra functions that only exist while the tests are running to allow increased test coverage of conditions callers can't directly cause themselves, but nonetheless can happen depending on other factors.