feat: introduce Cid::const_new, which allows accessing Cids from const contexts

[aatifsyed]

Resolves #138

Blocked on:

• chore(deps): update multihash requirement from 0.18.0 to 0.19.0 #137
• Merge and release of feat: introduce Multihash::new which can be called from const contexts rust-multihash#331
• Upgrade rust-cid to use the new release of the above

[vmx]

In case we really want to have const constructors (I'm not sure if it's really needed), what if we introduce our own Error::ReadError and Error::WriteError instead of the IO Errors? It would be a breaking change, but it would solve the problem in a cleaner way. What do others think about that idea?

[Stebalien]

I think we'd have to try it and see.

[aatifsyed]

Closing for the same reasons as multiformats/rust-multihash#331 (comment)