Add rustc::middle::cstrcache for getting c string bufs safely

We continue to leak string buffers in trans so this creates a way to get c
string buffers from strings while guaranteeing that they are not freed before
use.

Hopefully this can be made efficient in the istr regime.

src/comp/rustc.rc

@@ -30,6 +30,7 @@ mod middle {
     mod freevars;
     mod shape;
     mod gc;
+    mod cstrcache;
 
     mod tstate {
         mod ck;