Cleanup/merge rust-ipld related multiformats libraries

[Stebalien]

Motivation

We need a CID/multihash library for using both inside and outside of smart contracts, from rust. We currently have two options but neither of them are optimal.

Why Now

Unfortunately, these are base types around which we'll build the rest of the system, so we'd like to nail them down now instead of having to refactor later.

Done

We have a single multihash/cid library:

• Supports nostd.
• Lives in the github.com/multiformats org.
• Used by the ecosystem (i.e., PRs exist to update uses in forest & rust-libp2p).
• Ideally: Can be used without allocations.
• Ideally: Supports seamlessly abstracting over different sized CIDs.

Details

• All implementations:
  • Maximum size specified in the type (efficient in some cases, but it makes some patterns difficult).
    • WIP patch to support dynamically sized CIDs: https://github.com/yatima-inc/sp-multihash/pull/8
  • Allocations are often required for encoding. Probably not something we can easily fix, but we may be able to do something.
• Ours (github.com/multiformats/rust-*):
  • Poor nostd support (leading to large binaries).
  • Uses typenum instead of the new const generics.
• Yatima's (github.com/yatima-inc/sp-*):
  • Uses a special "bytecursor" library that requires allocations.
  • Can't read/write using io::Read/Write.
  • Is a separate implementation (not used by forest, libp2p, etc.).

[Stebalien]

1. Ours now support nostd.
2. We don't actually use nostd because it's just not worth it.