Custom memory allocation

I think I finally cracked the nut on custom memory allocation.  Back in
the ~0.3 versions, libcork had custom memory allocation, but it was very
invasive: you had to pass around lots of `cork_alloc` pointers all over
the place.  I dropped custom allocators to make the memory allocation
API much cleaner (and much closer to the standard `malloc` family of
functions).

This patch adds back in the ability to customize the details of how
memory is allocated, but without requiring lots of allocator pointers —
and most importantly, in a way that lets different sections of the
program have *different* custom allocators.  The key is the
`cork_current_allocator` macro, and in how all of the allocation
functions are defined static in a header file.  That allows different
source files to provide different values for `cork_current_allocator`,
which their copy of the static allocation functions will use.

There are two breaking changes.  The first is that you must now use
`cork_free`, `cork_cfree`, or `cork_delete` to free memory that you've
allocated, instead of the standard `free` function.  The second is that
you must keep track of how large each allocation is, and pass that in to
whichever free function you use.