Add a custom RTTI implementation for the AST (#1148)

* Add a custom RTTI implementation for the AST

Profiling was showing that the internal routines behind `dynamic_cast` were the worst offenders in the whole codebase, and a lot of this was being driven by casting inside the semantic checking logic.

This change takes advantage of the fact that we *already* had a custom RTTI structure built up for the classes of syntax nodes that was previously being used to implement string->class lookup and factory services to support "magic" types and modifier in the stdlib (e.g., the way that a modifier declaration in the stdlib just lists the *name* of a C++ class that should be instantiated for it).

That RTTI information already included a pointer from each syntax class to its base class (if any), based on the restriction that the AST node types form a single-inheritance hierarchy.
The existing code already had a virtual `getClass()` routine on AST nodes, and an `isSubClassOf` query on the class information.
Putting those pieces together to implement the `dynamicCast` and `as` routines was a cinch.

The work in previous PRs to layer an abstraction over all the existing `dynamic_cast` call sites and to support type-specific `dynamicCast` implementations inside `RefPtr<>` pays off greatly here.

* fixup: refactor implementation to appease more pedantic/correct compilers

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