Reduce str transmutes, add mut versions of methods.

src/libstd/lib.rs

@@ -296,6 +296,7 @@
 #![feature(stmt_expr_attributes)]
 #![feature(str_char)]
 #![feature(str_internals)]
+#![feature(str_mut_extras)]
 #![feature(str_utf16)]
 #![feature(test, rustc_private)]
 #![feature(thread_local)]