Add multiplexor capability to farray.__getitem__

Conflicts:
	pyeda/boolalg/bfarray.py
Change vs to more generic fs with term funcs
Lots of bfarray refactoring

Also, fix issue #76, performance issue with farray.__getitem__
Fixup issue with Function slice type
Maybe this actually works now
Miscellaneous refactoring
Use box(obj) instead of box(arg)
Use expr(obj) instead of expr(arg)
Get rid of deprecated operators
Add version 0.21 release notes
Release 0.21.0