Decouple test-vector types from the vm

[ZenGround0]

Currently the specs-actors reference vm implements its own types for generating and serializing test-vectors. We did this to get around the circular dependency (on lotus) that would be introduced by using the test-vectors repo types directly. These types and their methods make certain assumptions that are not good for general purpose use (i.e. https://github.com/filecoin-project/specs-actors/pull/1432/files#r647942788).

This is not ideal but ok as long as the actors types are conceptually coupled to the vm and not being used externally. However we are hoping to take on the responsibility of maintaining the format itself and there will be an issue describing the work around this soon. If that work materializes as expected the actors vm vector types will be strictly redundant and we should use the generally useable test-vectors types.

This issue tracks the work for moving the specs-actors reference vm from its own internal test vectors types to out-of-package general use types.