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/abi.rs

@@ -5,6 +5,7 @@
 
 use ir::{ArgumentLoc, AbiParam, ArgumentExtension, Type};
 use std::cmp::Ordering;
+use std::vec::Vec;
 
 /// Legalization action to perform on a single argument or return value when converting a
 /// signature.