Support reified generics for runtime type checking

[WillBAnders]

Following 04b3b94, runtime types should now force type erasure. This prevents many of the evaluators runtime checks from being meaningful, most notably with Struct properties (since existence is tied to the type).

Reified generics appear to be the best solution for this, which will likely require adding an invocation context to functions for storing runtime generic bindings. This context can also be used for storing the evaluator instance (rather than using a global variable) as well as debug information for errors in native functions with #3.