Fix IPv6 parse error stack overflow

Mac OS X has some better stack overflow detection, and it caught a
problem with the IPv6 parser.  A particular kind of parse error could
cause us to write an extra hextet into our temporary buffer, which we
hadn't allocated room for.  This patch adds an extra check to catch this
situation.
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.