Add support for bech32m (BIP350)

[pinheadmz]

Sources:

BIP 173 (bech32)

BIP 350 (bech32m)

• I used similar construction to secp256k1/ECDSA in the module exposure, i.e. bech32 is now a class whose constructor accepts an argument checksum (this is the only difference between bech32 and bech32m encodings, besides when they are used)
• This PR includes a commit https://github.com/bcoin-org/bcrypto/commit/63273b24829e971180d18b3069f383397d2b35f4 that I know belongs in libtorsion but it made more sense to me to develop the branch this way. I can port that over (and then re-import back to bcrypto) if approved
• This PR also includes commit https://github.com/bcoin-org/bcrypto/commit/9e774bd4d2e93cee1a856571db22cb5a7a007032 which fixes the test in legacy bech32. I'm not sure what happened but it looks like the push opcode offset 0x50 was confused for its decimal equivalent (80) which became 0x80. Looks to me like the test vectors from BIP173 were copied to bcrypto and then modified to pass the incorrect test. Luckily the real production logic for this is correct, in bcoin: https://github.com/bcoin-org/bcoin/blob/950e30c084141e7c8cb81233136b9e5bc7e1e02c/lib/script/script.js#L1592

[pinheadmz]

This branch now imports libtorsion from https://github.com/bcoin-org/libtorsion/pull/1 which is libtorsion current master branch plus the bech32 commit. Libtorsion has gone through a lot of changes since it was last imported and a good deal of the API expected by bcrypto was broken. All those fixes are in the last commit: https://github.com/bcoin-org/bcrypto/pull/61/commits/dc61f9d97fe92215f99ae7f9ebdeb8ea331117af