Implement multisig ADR

[ValarDragon]

This issue is for tracking this in the current project board. See #1960 for proposal.

[ValarDragon]

Ref #2164 (review)
What remains:

• threshold multisig:
  • complete switching everything to table driven tests (Threshold Multisignature implementation #2164 (comment))
  • add tests and document the recursive case (Threshold Multisignature implementation #2164 (comment)).
  • Adapt ADR to note the recursive case is allowed
• Implement weighted multisig
• Consider using compact bit array marshalling for marshalling the multisignature object.
• Make the compact bit array JSON elements and struct element names align.
• Align the "endianess" of the cmn.BitArray with the CompactBitArray (crypto: Add compact bit array for intended usage in the multisig #2140 (comment))

[alexanderbez]

Note, getting a Pubkey back from a bech32-ified string does not work in the Cosmos SDK because the bech32 string is too long (> 90 chars).