Get rid of some unnecessary imports
s/bitvec/exprvars/
Enhance bfarray docstring with name info
Add simple farray __str__ method
Add braces to avoid dangling else
s/int2array/int2farray
Replace 'cat' with 'fcat'

Avoids conflicting with IPython's builtin 'cat' alias.
Cleanup some lint warnings
Use Function.box methods to avoid const imports

It was non-ideal to import BDDZERO, EXPRZERO, etc.
This looks slightly cleaner, and avoids dependencies.