In the interest of making the library usable for both
testnet and mainnet without recompilation, static resolution
of network parameters has been replaced with a parameter passed
to the relevant functions. This also moves addres prefix constants
into the network parameters.
This enforces the MUST requirement in ZIP 32. A panic is used instead of
an error because this should be considered an implementation error.
Ideally the type system would prevent this from occurring at all.
Closes#125.