Support generalized += style operations on Vars

[dkoeplin]

(Spawned from Spatial Issue #113)

Ideally, when seeing x [op]= y, we should virtualize in the same way that scala compiler itself works:
First try readVar(x) [op]= y (do the operation on the current value of the Var)
then, if that's invalid, try x = readVar(X) [op] y (update the var itself).

Not sure if this is possible yet with scala-virtualized.