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.
whitequark