bech32 in go

bech32 is an encoding scheme by Pieter Wuille and Greg Maxwell described in a yet-to-be numbered BIP (https://github.com/sipa/bech32/blob/master/bip-witaddr.mediawiki)

It's a nice improvement over the original base58check encoding used in Bitcoin.

This code is still in progress but should be compatible with the C and python implementations linked in the BIP.