evaluate and improve array primitives

[JeffBezanson]

I suspect many of our critical array functions like ref, assign, cat, and transpose are highly suboptimal in many cases. For example ref/assign should be able to use memcpy (or equivalent) for the inner loop even for N-d.
We should time each of these functions in various cases and deal with any problems.

[JeffBezanson]

See commit 2450746 and commit 288ab97.
This is the kind of thing we will need. They can be improved even more using memcpy, writing specialized code for more specific cases, hoisting index computations, specializing areduce for various functions, etc.

[JeffBezanson]

ref, assign, reduce, and permute are now in pretty good shape. transpose needs a better algorithm. find has been improved but needs evaluation and tweaking. i think cat is ok but we need perf tests.

[JeffBezanson]

Status update: test/cat_perf.j has the start of some perf tests for cat. Probably the #1 thing left to do in this issue is use memcpy or copy_to in ref and assign, which cat will benefit from. hvcat also seems to have a bit too much overhead in the case of small (e.g. 5x5) arrays.