Add overload to Bech32.decode to handle arbitrary string input

[webmaster128]

Before this change, it was not possible to decode an arbitrary string from user input without preprocessing it for the library. The reason is we don't always have the strong type ${Prefix}1${string} or even ${string}1${string} available. A cast from string to ${string}1${string} is unsafe without an extra runtime check.

Even the tests file in this project mark the use of string inputs as type errors:

But it turns out genBech32 has the overload and runtime checks already. It just needs to be exposed to the user.

Closes #43