Clean-up type variable scope and binding

[ilevkivskyi]

Currently the code around type variable binding in functions is a bit unclear and does some strange things like binding typer variables before function type is analyzed and depending on TypeVarQuery returning short (unqualified) names.

[ilevkivskyi]

Another related comment here is that we have a bit too many "things" related to type variables (in addition to TypeVarScope mentioned above):

• TypeVarExpr
• TypeVarDef
• TypeVarType
• TypeVarId
  We can probably simplify a bit the model of how type variables work.

[ilevkivskyi]

Probably it makes sense to clean up (or at least document) various things related to type variable scopes while we are working on the new semantic analyzer. I just discovered an inconsistency in new analyzer, it was using full type variable name where short one was expected. Without documentation, such errors are easy to make.

[ilevkivskyi]

FWIW I more happy now with how type variables work (e.g. we use namespaces everywhere). There still may be bugs, I think tvar_scope.py is somewhat over-complicated, but we can handle issues on one-by-one basis now.