Switch MethodNum and ActorID to aliases

[austinabell]

Summary of changes
Changes introduced in this pull request:

• extension from Change ChainEpoch and TokenAmount types #309 there are some types that are starting to be used in Actors that don't yet have a reason to be their own type, and switching because it shouldn't be until there is an actual need. Makes referencing other actor's methods and calling much cleaner as well as comparisons/conversions.

Reference issue to close (if applicable)

Closes

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