New custom memory allocators.
This version doesn't require any mutex and is somewhat faster for lots
of allocations. Valgrind now shows a whole lot of "possibly lost"
warnings but they disappear when --enable-mem-debug is passed to
configure. I noticed that when using the old allocators that Valgrind
has trouble detecting some errors that are discoverable with
--enable-mem-debug. So we'll just do --enable-mem-debug to check for
errors.
This was a suggestion by Justus Winter [1].
1. https://lists.gnupg.org/pipermail/gnupg-devel/2016-August/031406.html