GADTs + inline records for more efficient doubly linked lists

[mmottl]

This branch implements the same kind of efficiency improvement as the one recently submitted for the union-find algorithm. Though the patch touches many lines, the vast majority of changes are completely trivial and easily recognized as correct.

The patch will require the upcoming OCaml 4.04 (or beta) to show performance improvements. It eliminates an indirection and associated memory allocations. In particular, changes to the first element of the doubly linked list (e.g. the frequently used insert_first) will now need one less allocation. Only the first function is slower now, which is likely insufficient reason not to use the patch.

I don't have a comprehensive benchmark suite (you probably do), but in some simple tests (e.g. insert_first of a lot of elements + removal of each inserted element), the performance gain was around 2.5% and allocated words were reduced by about 10%.

The speed improvement isn't overwhelming, mostly because the OCaml runtime is so crazy good at small allocations. But a reduction in allocated words will reduce GC pressure and possible latencies associated with more frequent collections.

[yminsky]

See comment on #59

[hhugo]

@mmottl, would you mind rebasing you PR ?

[mmottl]

@hhugo No problem, it's rebased now.

[yminsky]

This seems like a totally sensible improvement. Is there any reason why we haven't imported this feature?

[yminsky]

CC @bcc32 @rnml @ceastlund