Add bech32_mod for double public key encoding/decoding

[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.

[aguycalled]

LGTM
I would also add support for encoding/decoding from/to blast::DoublePublicKeys in the class DestinationEncoder and the function DecodeDestination located in the key_io.cpp file. Although we can do it in a separate PR too if you prefer.

[gogoex]

Sure. I added functions to encode/decode a double public key to key_io.cpp. Since DestinationEncoder is not exposed to outside, I added the functions directly under key_io.cpp.

[mxaddict]

Is this PR ready for final review?

[gogoex]

@mxaddict Yes. Sorry to the late response.
@aguycalled Regarding the key_io.cpp change, can we do it a separate PR?