In this commit, we add an additional field to the ErrInvalidChecksum,
the bech32m version of a checksum. When decoding, we don't now what
version they actually _intended_ to use, so we'll opt to include both
checksums to aide in debugging and error reporting.
This commit brings a host of improvements to the bech32 package. The
public interface of the package remains unchanged.
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.
