Replace verify_signature with verify_bls_aggregate

[Stebalien]

We currently have:

• verify_signature for verifying arbitrary signatures.
• recover_secp_public_key for recovering a secp public key from a secp signature.

Unfortunately, the first one is a bit limited:

• It does hashing internally, which restricts the user to a specific hash function.
• It needs the public key to do verification in the first place, which is why we needed recover_secp_public_key.
• It can't, e.g., validate bls signature aggregates.

Proposal:

1. Add a new verify_bls_aggregate (where the aggregate may have one element).
2. Remove verify_signature.
3. Move all hashing to userspace.

[Stebalien]

NOTE: this needs a FIP/discussion, but I'm posting the issue here because it'll be important for M2.2.

[Stebalien]

ALSO NOTE: The correct way to validate an "account's" signature is to use FRC0044.

[cryptonemo]

Hey @Stebalien , Jake and I are looking at this this week. Do we need FFI wiring for this API too, or are you wrapping it internally or something?

[Stebalien]

We need a new syscall (wasm -> FVM) but nothing in the FFI (lotus -> FVM).

[Stebalien]

Fixed in #2003 (although we haven't removed the old version).