We always want to crawl the entire stack, and it's
possible to determine the extents of the stack, so just
do a scan over the entire memory range.

Also, change the way the interpreter keeps track of its roots;
we don't really need to associate the roots with a specific
interpreter frame.

This should hopefully clear up the weirdness about libunwind
trying to unwind through the pthreads assembly code, and potentially
also make stack crawling faster.