Split types into separate symbol table and type-level functions

[krishnachittur]

This PR refactors types in order to make future improvements/additions to typing either. In particular, we

1. split off a separate type-level symbol table for looking up type identifiers (e.g. names of structs, enums, aliases)
2. break up Type into BaseType and ResolvedType, which allow us to cleanly express type-level functions like list, map, etc.
3. store trees of type identifiers in the symbol table (e.g. map[string, list[Foo]]) to deduplicate types

In the longer term this should also unblock work on generic user-defined types.

TODO (for this PR)

• Fix tests
• Add new tests and verify parser

TODO after this PR

• update StructDecl parsing to mutate in place in the type symbol table
  • add the newly defined Struct type to the type symbol table immediately after parsing attrs
  • each parsed function goes into the term symbol table (e.g. keyed Cat::meow, value fn[...]) AND the type symbol table (as a mutation of e.g. the Cat StructDecl)
    • function decl args get typed with type identifiers like everything else, so we can refer to a struct with "self" even if that struct gets mutated later
• add uses_self to BaseType::Fn so that we can track those kinds of functions in the symbol table
• handle ambiguous attribute access vs method call (e.g. cat.meow() could call a closure attribute, not just a method)
  • prioritize method calls when they exist, then access attributes
• add struct default values to BaseType