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/verifier/mod.rs

@@ -73,6 +73,8 @@ use std::collections::BTreeSet;
 use std::error as std_error;
 use std::fmt::{self, Display, Formatter, Write};
 use std::result;
+use std::vec::Vec;
+use std::string::String;
 use timing;
 
 pub use self::cssa::verify_cssa;