Add an Interface view of Packages

[johnynek]

Currently, when we typecheck and render fully typed ASTs (TypedExpr), we require all the fully typed packages of all the imports.

We should be able to have an interface view that only lists the values and types exported by a package, without needing the fully typed ASTs.

The value of this is to cement in the design that we can do this separately, and as long as an interface does not change, there is no need to rerun a package typechecking.

This becomes really nice when coupled with hashing of the API or when used with build systems such as bazel that can make compilation only depend on the API artifacts, and not the fully typed ASTs.

For this, we would likely have a linking stage at the end that converts fully typed ASTs into some other output (JSON, Yaml, skylark, JVM class files, C, etc...)

[johnynek]

done in #162 ... no serialization yet, but that is probably a subissue of #54