Slab alloc threadcache

[dsm9000]

This is a skeleton of the GC thread cache ( https://github.com/snazzy-d/sdc/issues/269 )

To become useful, it still needs:

• ~Bulk alloc of slab slots in the refiller~ (done)
• Sort on "closest to vacant slab state" prior to eviction when flushing
• Tests.
• Possibly, other corrections.

The cache bins are double-ended queues. The rationale for this:

• We don't want to sort in getSlot
• If we intern all returned allocs on the same end we get new ones on, it becomes impossible to guarantee correctness of https://github.com/dsm9000/sdc/blob/master/sdlib/d/gc/tcache.d#L768 , https://github.com/dsm9000/sdc/blob/master/sdlib/d/gc/tcache.d#L740 , and all similar tests.
• It would be nonsensical and breaking with traditional allocator behaviour for new allocs to "walk backwards" pointerwise.
• Makes the eviction sort (TODO) easier to implement efficiently.