After going through the MODE=dbg and MODE=zero build modes, a bunch of
little issues were identified, which have been addressed. Fixing those
issues created even more troubles for the project, because it improved
our ability to detect latent problems which are getting fixed so fast.
This change introduces support for Linux-style uc_context manipulation
that's fast and works well on all supported OSes and architectures. It
also integrates with the Cosmpolitan runtime which can show backtraces
comprised of multiple stacks and fibers. See the test and example code
for further details. This will be used by Mold once it's been vendored
