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/middle/cstrcache.rs

@@ -0,0 +1,29 @@
+// Provides a safe way to get C pointers to strings, because in many
+// places LLVM wants a string but doesn't take a copy.
+
+import std::str;
+
+export t;
+export mk;
+export get_cstr;
+
+type t_ = @{
+    // This string is boxed so that I remember that it has to be boxed
+    // after the ivec conversion.
+    mutable cache: [@str]
+};
+
+tag t {
+    private(t_);
+}
+
+fn mk() -> t {
+    ret private(@{mutable cache: []});
+}
+
+fn get_cstr(t: &t, s: &str) -> str::rustrt::sbuf {
+    let boxed = @s;
+    let buf = str::buf(*boxed);
+    (*t).cache += [boxed];
+    ret buf;
+}

src/comp/rustc.rc

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