Hi.
This patch add support for bitcasts in libgccjit.

It passes the JIT tests, but since I added a function in tree.c, I
wonder if I should run the whole testsuite.

David, you can now disregard my question in my email about 128-bit
integers regarding my issue with initialize_sizetypes being called
multiple times because this patch fix this issue.
I turns out there was a cache of types that needed to be cleared when
you initialize the JIT.

The check for sizes is pending, because it requires the updates to
get_size I made in my patch for 128-bit integers.

Thanks for the review!