Protocol support

[JelleZijlstra]

• Fix 3.7
• Extract protocols from stubs
• Make protocol bases from typeshed work
• Test against internal codebase
• Profile to see if some additional caching is valuable

[JelleZijlstra]

I added a LazyValue to help with some recursive protocols; e.g. typing.Iterable and typing.Iterator are mutually recursive. This has become complicated enough that I'm going to try to extract some smaller parts and land them separately.

[JelleZijlstra]

Current status:

• Need to fix one more test
• Incompatible argument type for actual: expected Container[Any] but got str still happens

[JelleZijlstra]

I've been sitting on this for a while and I think the approach in this branch (a new Value class) won't work well.

What I'd like to do instead:

• TypedValue can contain either a type or a fully-qualified reference to a type defined in a stub.
• We lazily create something like the current TypeObject from that type or reference. The type object contains flags for whether it is a structural or a nominal type, and further information for types defined only in stubs (like the method dictionary).
• Creating the TypeObject will require a typeshed client, so we do it lazily in can_assign().
• Applying typevars should probably make a new TypeObject.

[JelleZijlstra]

#295 implements a basic version of the above (only for stub-only protocols for now). Closing this PR (which is full of merge conflicts anyway).