Add the 'M' and 'm' MALLOC_OPTIONS flags, which control the maximum medium size
class.
Relax the cap on small/medium run size to arena_maxclass.
Reduce arena_run_reg_dalloc() integer division code complexity.
Increase the default chunk size from 1MiB to 4MiB.
jemalloc is configured.
Modify arena_malloc() API to avoid unnecessary choose_arena() calls. Remove
unnecessary code from choose_arena().
Enable lazy-lock by default, now that choose_arena() is both faster and out of
the critical path.
Implement objdir support in the build system.
