Migrate type-checker to native types and declarations

[TristonianJones]

Migrate the type-checker to native types and declarations

This change simplifies the internals of the type-checker considerably by relying
on function overload collision checks to happen within the decls.FunctionDecl
instances on Merge. Additionally, the native types have replaced the protobuf
type declarations used throughout the checker previously.

The files with the most non-trivial changes are the checker/checker.go and
checker/types.go. When reviewing the code, please consider these changes
carefully.

Only one location continues to use the proto-based exprbp.Type, and that's
the ref.TypeProvider which will be updated in a follow-up PR to complete
#568

[jcking]

protobuf Get accessors protect against nil dereferencing by returning default values. Here we switch to direct accessors. Hopefully there are not places where we accidently accepted nil and got a default string value, where now we will dereference. We should probably add more exhaustive checks to ensure nil doesn't get placed into the environment accidently.