Move lexical environment to an easily accessible location

Problem

To start work on refinement types, we probably want the lexical environment to be easily accessible, especially from type-rep.

Approach

Add a new dynamic variable in util-vars and change existing dereferences of lexical-env to a function somewhere.

Code review: CTYP-248
Patch: move-lex-env.patch
Commit: CTYP-248
To appear: 0.3.8