Add bech32_mod to encode/decode 96-byte double public keys

[gogoex]

This is an intermediate step to address the first 2 tasks in issue #122:

• Create a modified version of bech32 which allows the encoding of two public keys + prefix.
• Document it in https://github.com/navcoin/navcoin.github.io/tree/main/sources/docs

This change adds bech32_mod::Encode that is designed to encode 165-byte 5-bit input vector (after 8-bit to 5-bit conversion and inserting 1-byte separator, 96-byte double public key with 2-byte HRP becomes 165-byte long), and bech32_mod::Decode that is capable of perfectly detecting up to 5 errors with helper functions to convert betwen 8-bit based vector to 5-bit based vector and vice versa.

The process of finding a generator polynomial that is capable of the desired detection capability is discussed in doc/bech32-mod-gen-poly in detail.