Duplicate fvm_shared::actor::builtin::Type into runtime::builtins::Type

[anorth]

This PR copies the builtin actor Type enum out of the FVM SDK into the built-in actors repo.

Breaking coupling is necessary to enable development of the built-in actors (e.g. adding a new one) without depending on changes to the FVM SDK. Since the FVM doesn't want to depend on the actors, any changes to the enum need to be propagated to the FVM.

This removes all but one reference to the path fvm_shared::actor, the holdout being the conversion between the two Type enums at the SDK syscall. This last one should be removed by changing the SDK to pass through the bare i32 value that it wrapped.

[~/p/f/builtin-actors](anorth/builtin-type)$ rg 'fvm_shared::actor'
runtime/src/runtime/fvm.rs
173:        let t = fvm_shared::actor::builtin::Type::from_i32(typ as i32).unwrap();

Closes #520.

[ZenGround0]

Can't speak to the implications on fvm side but for builtin actors this change LGTM