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.
This closes an old and horrible issue in that some code in ksupport
may implicitly try to allocate and we'll never know until it crashes
at runtime inside liballoc_stub.
This also removes liballoc_stub.
This also removes ReadExt and WriteExt from libproto.
Hooray!
