Add ToCStr method .with_c_str()

.with_c_str() is a replacement for the old .as_c_str(), to avoid
unnecessary boilerplate.

Replace all usages of .to_c_str().with_ref() with .with_c_str().

src/libstd/unstable/dynamic_lib.rs

@@ -66,7 +66,7 @@ impl DynamicLibrary {
         // T but that feature is still unimplemented
 
         let maybe_symbol_value = do dl::check_for_errors_in {
-            do symbol.to_c_str().with_ref |raw_string| {
+            do symbol.with_c_str |raw_string| {
                 dl::symbol(self.handle, raw_string)
             }
         };
@@ -145,7 +145,7 @@ mod dl {
     use result::*;
 
     pub unsafe fn open_external(filename: &path::Path) -> *libc::c_void {
-        do filename.to_c_str().with_ref |raw_name| {
+        do filename.with_c_str |raw_name| {
             dlopen(raw_name, Lazy as libc::c_int)
         }
     }

src/libstd/unstable/lang.rs

@@ -29,7 +29,7 @@ pub fn fail_bounds_check(file: *c_char, line: size_t,
                          index: size_t, len: size_t) {
     let msg = fmt!("index out of bounds: the len is %d but the index is %d",
                     len as int, index as int);
-    do msg.to_c_str().with_ref |buf| {
+    do msg.with_c_str |buf| {
         fail_(buf, file, line);
     }
 }