Fix memory leak by freeing boxed value.

Which requires defining our own way to free a boxed value.  This doesn't
feel quite right, unless we're *really* supposed to be using GC.
Use githwxi/ATS-Postiats as upstream and do a full bootstrap

Github upstream seems to be the authoritative upstream, so we should
switch over.  It looks like CBOOT wasn't dropped completely, so we might
be able to shorten this compile process in the future, but this does
serve as a good record of how to do a full bootstrap.