QIR Runtime should support alternate or custom target packages

[swernli]

Right now, the QIR runtime is built using the explicit list of gates from the QSharp.Core target package, as seen from the definition of the IQuantumGateSet in the public header QuantumApi_I.h. Because of this, any gate sets that don't have the same list of intrinsics will need to either copy and modify the runtime files or provide stubs for the core set and add additional ones. This can be especially problematic if the alternate gate set defines gates of the same name but with different signatures. We need to separate the gate set from the math related intrinsics and the runtime support such that a generic, gate set agnostic runtime library can be produced and added to a gate set definition to complete the implementation.