Support for conditionals at any level of looping

[lynlevenick]

It would be nice to be able to write a conditional at any location in the expression, e.g. c![a + b, for a in some_vec, if a % 5 < 2, for b in 0..10]. This could work well with generators (#1) to reduce nested computations, though I'm not sure how well that would end up vectorizing if at all.