Add AchillesHeel to expression parsing
Cleanup some unnecessary dependencies
Update table/bdd top matter
Implement new pyeda.boolalg.bfarray module

Partially addresses issue #68, which proposes a new Boolean function
array type.
Mega function array additions/cleanup

* s/FunctionArray/farray
* Get rid of expr/bdd/tt-array functions
* Simplify farray constructor
* Add several operators to farray
* Use Function instead of None for empty array
Patch new yield from statement