malloc: send freed small chunks to smallbin

malloc: send freed small chunks to smallbin

Large chunks get added to the unsorted bin since
sorting them takes time, for small chunks the
benefit of adding them to the unsorted bin is
non-existant, actually hurting performance.

Splitting and malloc_consolidate still add small
chunks to unsorted, but we can hint the compiler
that that is a relatively rare occurance.
Benchmarking shows this to be consistently good.

Authored-by: k4lizen <k4lizen@proton.me>
Signed-off-by: Aleksa Siriški <sir@tmina.org>