This changes a few things around binding logic, as part of eliminating
all of the legacy logic and weaving it into the new codebase:
* Give Scopes access to the Context object. Related, add a TryRegister
method to Scope that is like RequireRegister, except that instead of
fail-fast upon encountering a duplicate entry, it will issue an error.
* Move all typecheck visitation functions out of the big honkin' switch
and into their own member functions. As this stuff gets more complex,
having everything in one routine was starting to irk my sensibilities.
* Validate that packages have names.
* Store both the package symbol, plus the canonicalized URL used to
resolve it, in the package map. This will help us verify that versions
match for multiple package references resolving to the same symbol.
* Add nice inquiry methods to the Class symbol (Sealed, Abstract, Record,
Interface) that simplify accessing the modifiers on the underlying node.
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