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/condcodes.rs

@@ -266,6 +266,7 @@ impl FromStr for FloatCC {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use std::string::ToString;
 
     static INT_ALL: [IntCC; 10] = [
         IntCC::Equal,