Add support for globals which don't get captured

Add support for globals which don't get captured

Lots of top-level definitions don't need to be copied into every
closure, because they're constant for the whole program. Global
variables are never inserted into the upvars entry, so they won't be
copied into closures.

This also adds "builtin" names that are inserted into the initial
context for arithmetic and comparisons. Booleans have to be builtin due
to being returned by the comparisons, but they're still intended to be
implemented via church encoding.