[ruby-core:64614] cowspace (work-in-progress)

Hi all, I started working on a cowspace branch.  Based on the mspace API
of new versions of dlmalloc.   The idea is to have a separate malloc
space for long-lived, WORM (write once, read many) data which may
increase memory sharing for forked processes.

The mspace API is similar to normal malloc:

	malloc(size) => mspace_malloc(GET_VM()->cowspace, size)
	free(ptr) => mspace_free(GET_VM()->cowspace, ptr)
	...

There are rb_cow_(malloc/free/...) function wrappers as well
as COW_(ALLOC/ZALLOC) macros to ease migrations.

However, I haven't found any benefits, yet.  _Maintaining_
CoW-friendliness is difficult in long-term so it might not be worth it
(compared to overall memory reductions).

http://bogomips.org/ruby.git/log/?h=cowspace
(git clone git://bogomips.org/ruby.git / branch: cowspace)