Type checker

[edwardpeters]

Adds support for more real type checking to evaluation process.
Also did a lot of cleanup and minor fixes. A couple more substantial changes were called for:

-A much more complete (though still not 100%) type checker now runs on the actual IR node to be evaluated, immediately prior to evaluation. This may be turned off through the RTExecutionContext.Options passed to the evaluation's ZPure env.

• All evaluator entry points other than the single-arg "IR" function now take a param and a varargs of param. This lets you call the evaluator with multiple arguments. It does preclude calling an entry point defined as an FQName with no arguments, but I don't think that's a real use case (and support could be added if necessary)
• (@DamianReeves ) A duplicate definition of MorphirRuntimeError was causing namespace issues. I think it was only there as a placeholder for in-development ZPure work, so to avoid the namespace conflict I renamed that error type to RTError and wrapped its members in an object.