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/regalloc/live_value_tracker.rs

@@ -13,6 +13,7 @@ use regalloc::affinity::Affinity;
 use regalloc::liveness::Liveness;
 use regalloc::liverange::LiveRange;
 use std::collections::HashMap;
+use std::vec::Vec;
 
 type ValueList = EntityList<Value>;