Chunk better

[treeowl]

parListChunk previously split a list up into chunks, applied
the given strategy to each chunk, and then put them all together
again. This led to two extra copies of the list.

We get very little benefit from actually splitting the list, because the
parallel computations need to traverse their part anyway; we can instead just
hand off the whole list and let them count out their chunk. We count each
chunk twice, but that shouldn't cost enough to matter.

Now that Eval has a MonadFix instance, we can avoid actually having
to put together lists at the end; instead, we pass each parallel
computation the (as-yet-uncomputed) result of calculating the rest
of the list.

[simonmar]

Great. Can we have some tests and benchmarks please?