Add support for medium size classes, [4KiB..32KiB], 2KiB apart by default.

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.