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/licm.rs

@@ -8,6 +8,7 @@ use dominator_tree::DominatorTree;
 use entity::{EntityList, ListPool};
 use loop_analysis::{Loop, LoopAnalysis};
 use timing;
+use std::vec::Vec;
 
 /// Performs the LICM pass by detecting loops within the CFG and moving
 /// loop-invariant instructions out of them.