Representing built-in types in the AST (Symbol and Ref)

[elegios]

Most of our built-in types have their own AST node (e.g., TyInt, TyFloat, and TySeq) but some are represented by TyCons (at least Symbol and Ref). Before #902 these were distinguished by being left unsymbolized, after #902 they're distinguished by having particular globally set Names. This seems inconsistent.

There are a few ways to approach this:

• Give Symbol and Ref their own AST-nodes.
• Represent all built-in types with globally reserved Names. A minor variation of this would be to only do this for atomic types (TyInt, TyFloat, etc.) and not composite ones (TySeq).
• Make Symbol and Ref externals, not built-ins. These types have relatively little need for special casing in the compiler, and what need there is should be doable anyway (e.g., via something like _getTyConExn added in Loader, generate-json-serializer, generate-pprint, generate-eq, and generate-utest #901).

There is also an orthogonal, but related concern: Int, Float, etc., are reserved keywords in the parser today. This could be changed, make it so that Int, Float, etc., are type aliases in scope when the program starts.