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/dominator_tree.rs

@@ -8,8 +8,8 @@ use packed_option::PackedOption;
 use std::cmp;
 use std::mem;
 use timing;
-
 use std::cmp::Ordering;
+use std::vec::Vec;
 
 // RPO numbers are not first assigned in a contiguous way but as multiples of STRIDE, to leave
 // room for modifications of the dominator tree.