Rework the design a bunch

[msullivan]

• Lots of renames
• Make Iter over fixed-length tuple types the core iteration operator
• Move most things into function/type alias tables from syntax.
• Make Member (was Property) just a regular parameterized type.
• Use GetAttr/GetType as accessors for Attrs. They are defined
  just as type aliases that call GetArg!

I had to add a bunch more eval_typing calls to typing.py in order
to keep things working once we were using helpers defined as type
aliases.

(I renamed DirProperties to Attrs for now. The version that includes
methods will be Members maybe?)