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/isa/riscv/settings.rs

@@ -12,6 +12,7 @@ include!(concat!(env!("OUT_DIR"), "/settings-riscv.rs"));
 mod tests {
     use super::{builder, Flags};
     use settings::{self, Configurable};
+    use std::string::ToString;
 
     #[test]
     fn display_default() {