Use malloc / free instead of mmap for stacks.

mmap is not a super fast operation due to the fact that it has to modify the
processes VMA. This can be slow if it's done a lot. Additionaly, system malloc
generally has some better properties (like caching common allocations) so you
get stack pooling for pretty much free. tcmalloc or jemalloc might be able to
do even better.

Also added simplehttpfileserver example app taken from cmpxchg16's fork so I
can compare against that one.