Decouple actor Type enum from the FVM

[anorth]

The built-in actors have a coupling to some FVM internals through the existence of a Type enum over the built-ins. This prevents adding new ones without making changes in FVM first. This blocks development of the datacap token and FIL+ claim actors for FIL+ decoupling.

Current proposal for fixing this is to drop the import and just copy the Type codes into the builtin-actors repo (with a mapping from syscall results at the runtime). This would introduce a bit of duplication to FVM, but pretty scoped. We can propagate the changes there after development.

Long term we should hope to remove this concept entirely. Most of the uses are anachronistic or can be expressed another way (e.g. use singleton addresses, look up miners in power actor state)