Add loop operator in fcode

[radrow]

Added loop construction in fcode to optimize list comprehensions and sequencing operator (and get rid of implicit imports)

[hanssv]

Any numbers on how much better this is?

[radrow]

Good point Hans, recent progress in REPL allowed checking that after my """fix""" it uses 3x more gas... Although I think it's a performance bug in my definition of .., not in the idea of loop itself. Or maybe I messed up the optimizer?

[radrow]

Maybe it's because of the tuple?