Introduce numeric names
Move Eval monad into PTS.Syntax.Names

This monad is in fact just a freshness monad. I'm going to rename it
next, and then replace the implementation.
Encapsulate Eval more: hide envToNamesMap
Hide original names into NumberNames...

...without using them as keys ever.
Replace Eval state with an Int for freshness

As usual, this breaks on bigger files.