bech32: back port improvements from decred/dcrd

[onyb]

What is this about?

This PR brings a host of improvements to the bech32 package, back porting from the following commits:

• decred/dcrd@9b88dd0
• decred/dcrd@3a0dce1
• decred/dcrd@3cfb088

The public interface of the package remains unchanged, of course not including new public methods.

Summary of changes

• Improved error handling using dedicated error types. Programmatically detect if the errors produced are the expected ones.
• Improve test coverage to test more corner cases. Added test vectors from Bitcoin Core.
• Add a benchmark for a full encode/decode cycle of a bech32 string.
• Add a new function DecodeNoLimit, for decoding large bech32 encoded strings. It does NOT validate against the BIP-173 maximum length allowed for bech32 strings.
• Automatically convert the HRP to lowercase in Encode function.
• Improve performance of Encode/Decode functions by using strings.Builder.
• Improve memory allocation in ConvertBits function.
• Updated documentation.

Credits: @matheusd

Closes #152 and #168.

[onyb]

Oops, Travis CI was testing against really old versions of Go. Additionally, gometalinter seems dead, and I don't really want to fix all the CI stuff in this PR. Let's have #167 merged first.