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/immediates.rs

@@ -652,6 +652,7 @@ mod tests {
     use std::{f32, f64};
     use std::str::FromStr;
     use std::fmt::Display;
+    use std::string::ToString;
 
     #[test]
     fn format_imm64() {