FVM crate extraction/refactor

[Stebalien]

Motivation:

• Abstracting over multiple FVM versions is somewhat painful because some "common" types live in this repo. This is requiring quite a few annoying and otherwise unnecessary conversions in the FFI.
• Changes/enhancements to our "ipld" types require changes to this repo.
• Changes to, e.g., the "token amount" type only apply to a single FVM version.

Note: We don't have to be concerned about accidentally changing actor behavior, because actor behavior is fixed on each network upgrade (when we compile to wasm). That gives us a lot more flexibility to iterate on some of these datastructures.

General idea:

• Move everything in "ipld" into a new repo (fvm-ipld-structures? or maybe just fvm-datastructures? or fil-something?). We could do multiple repos, but that probably isn't worth it.
• Move TokenAmount and Address from "shared" to their own crates. We can import these in fvm_shared, but pulling them into a new crate will make it easier to abstract over multiple versions (and iterate).
• Try to get rid of "raw bytes" in interfaces. This is basically just a Vec<u8> with some other features, but it pollutes and complicates some interfaces (feat: remove RawBytes from APIs #965).

[Stebalien]

cc @anorth and @vyzo

[vyzo]

It was a PITA to do the conversion and I am not looking forward to doing it again in the future, so in principle I support this move.

Implementation wise it might be a little tricky and quite timr consuming.

[anorth]

Thanks very much for starting this. Perhaps we can start with a small extraction, learn from that, then continue? The shared types might be a good starting point.

[Stebalien]

Honestly, the easiest extraction would be the entire ipld subfolder. But I'm also happy to start by extracting address, or something like that.