Simplify tcache object caching.

Use chains of cached objects, rather than using arrays of pointers.

Since tcache_bin_t is no longer dynamically sized, convert tcache_t's
tbin to an array of structures, rather than an array of pointers.  This
implicitly removes tcache_bin_{create,destroy}(), which further
simplifies the fast path for malloc/free.

Use cacheline alignment for tcache_t allocations.

Remove runtime configuration option for number of tcache bin slots, and
replace it with a boolean option for enabling/disabling tcache.

Limit the number of tcache objects to the lesser of TCACHE_NSLOTS_MAX
and 2X the number of regions per run for the size class.

For GC-triggered flush, discard 3/4 of the objects below the low water
mark, rather than 1/2.