Previously, small allocations which were sampled as part of heap
profiling were rounded up to `SC_LARGE_MINCLASS`. This additional memory
usage becomes problematic when the page size is increased, as noted in #2358.
Small allocations are now rounded up to the nearest multiple of `PAGE`
instead, reducing the memory overhead by a factor of 4 in the most
extreme cases.
Add new runtime option `debug_double_free_max_scan` that specifies the max
number of stack entries to scan in the cache bit when trying to detect the
double free bug (currently debug build only).
