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/simple_gvn.rs

@@ -6,6 +6,7 @@ use dominator_tree::DominatorTree;
 use ir::{InstructionData, Function, Inst, Opcode, Type};
 use scoped_hash_map::ScopedHashMap;
 use timing;
+use std::vec::Vec;
 
 /// Test whether the given opcode is unsafe to even consider for GVN.
 fn trivially_unsafe_for_gvn(opcode: Opcode) -> bool {