I committed the fix for bug #19 without a test because I didn't know
how our test worked yet; here's a test.

A somewhat more desirable way to do this would be to commit the
test *first*, marked as "incomplete" with `g_test_incomplete()` (an
expected failure).  However, `g_test_incomplete()` does not handle
segfaults!  There's no way to mark a segfaulting test as an "expected
segfault".  So if you want to verify that this test reveals the bug,
you'll need to `git show thiscommit | patch -p1` or something, in a
tree that doesn't have the fix applied.  Or you can comment out the
fix, I guess.

Storing the size of the blocks is now only activated when DEBUG__MEMFILL
has been defined.

We replace instances of void* w/ char* arithmetics.

This allows system_allocator.c to be compiled with MSVC.

Also we introduce a struct to represent the size_t header.

Refactored all of the desugaring code to no longer depend on memory being initialized to 0. Everything is about 12% faster now.