Add type import
Minor fixes; add test for inference.
More imports, normalize constructor names in type-structs
Use checking mode for let bindings with annotations.

If there's an annotation on a let binding, we should enter checking mode
to propagate more information. Also, generate constraints for the return
type of parametrized functions when in checking mode.
Add variant t-app to the Type type.
Make changes as recommended by @jpolitz.
Add t-app cases to methods, and other comparison related stuff