Sequential array operations

We have already got `sfoldl`, but we need more sequential array operators. In particular, we also need to be able to produce arrays in sequential code — e.g., we should have `smap` and `szipwith`. (I'm not convinced this is the best naming scheme, though.)

This leads to having `Exp` computations that are array-valued. The main conceptual problem with those is that we need to be able to statically track the size of these arrays (after all, an array-valued `Exp` result may be the result of a `map`). This may be possible with the type-level naturals that should be coming with GHC 7.8. They are not very powerful yet (reasoning-wise), but I think we can already do a lot with some very simple constraints. Our main cases seem to be that a function preserves the size or collapses an array to a singleton.

This might also be helpful for #142 as it would give us array stencils with static sizes.