Add use declarations for std features.

Merge the `use` parts of the `no_std` branch. This reduces the diffs
between master and the `no_std` branch, making it easier to maintain.

Most of these changes are derived from patches by @lachlansneff in
https://github.com/Cretonne/cretonne/tree/no_std.

lib/cretonne/src/ir/dfg.rs

@@ -944,6 +944,7 @@ mod tests {
     use cursor::{Cursor, FuncCursor};
     use ir::types;
     use ir::{Function, Opcode, InstructionData, TrapCode};
+    use std::string::ToString;
 
     #[test]
     fn make_inst() {