Provide utility function to convert a InstructionSetArchitecture to a CompilerISA

[notmgsk]

The compile function takes in a CompilerISA. On the client side, however, the InstructionSetArchitecture is the type that is more readily available (via QCS API). PyQuil knows how to convert between the two, but outside of pyQuil there is no such tool for conversion. So we should provide one.