This also updates / is a prerequisite for updating smoltcp.
Rationale for changes made:
* compiler_builtins is now shipped in the rust prefix.
* rustc's libpanic_unwind no longer works for us because it
has a hard dependency on Box (and it's a horrible hack);
fortunately, we only ever needed a personality function
from it.
* panic and oom handlers are now set in a completely different
way.
* allocators are quite different (and finally stable).
* NLL caused internal compiler errors in runtime, so code using
NLL was rewritten to not rely on it and it was turned off.
It doesn't work extremely well, and the results can be somewhat
confusing, and it needs a libunwind patch, but it's the best I could
do and it seems still useful.
