[T4 ONLY] BIP32 Wallet Compliance - Aggsig Updates - Bulletproof Updates

[yeastplume]

This is ready now (but needs to be kept up-to-date until we're ready for a T4 merge)

Contains:

• Aggsig enhancements for Add total public excess to Schnorr challenge #1423. Contained in rust-libsecp256k1 and Grin is now including the public-key sum total in e
• Aggsig signature hash being computed incorrectly Aggsig signature hash being computed incorrectly #1358
• rust-libsecp256k1 API provides ability to contain an optional extra key in the signature: Optional extra key in signature secp256k1-zkp#25 (this is not being used directly by Grin at present)
• Modifications to Bulletproof API updates to support multi-party bulletproofs Multi-party bulletproof secp256k1-zkp#24 (note this is contained in the libsecp256k1 at present, but not exposed in rust-libsecp256k1)
• Modification to Bulletproof API to support 16 bytes of arbitrary message (for BIP32 Compliance, below) mimblewimble/secp256k1-zkp@e6f1191

And updates the wallet to use BIP32 paths internally as per #1428. This includes:

• The wallet supports multiple named accounts, with each 'account' being a mapping between a label
  and a BIP32 'parent path'
• BIP32 'parent paths' are set according to recommendations in the BIP32 spec. The 'default' account parent path is 'm/0/0' and outputs for this account are given IDs 'm/0/0/1', 'm/0/0/2' etc... the next created account will have parent 'm/1/0' with outputs at 'm/1/0/0', 'm/1/0/1' etc
• Internal wallet identifiers are encoded 4-level deep BIP32 paths.
• BIP32 path is encoded in an output's bulletproof on creation, so it can be recovered along with the amount by rewinding with the original nonce, which is blake2(commit|secret_key(m), (secret key of being the root path m generated by the wallet seed/password)
• All outputs and transactions are created against the active wallet account, which defaults to an account called 'default' (so no change in usage if account functionality is ignored by the user)
• Commands and switches to create and switch active account for wallet commands + Documentation
• Wallet restore will restore all outputs as per the path stored in the bulletproof, and re-create accounts for any detected parent paths. Also no longer needs to grind to find keys, so should be much faster overall.

[yeastplume]

Done now. Modified top comment to reflect all of the changes contained in this PR.