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.
Because that's the current API — it returns a `const char *`.  If we want to change that to return a `char *`, we can (as a separate PR), but this doc accurately describes the function.