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/loop_analysis.rs

@@ -8,6 +8,7 @@ use flowgraph::ControlFlowGraph;
 use ir::{Function, Ebb, Layout};
 use packed_option::PackedOption;
 use timing;
+use std::vec::Vec;
 
 /// A opaque reference to a code loop.
 #[derive(Copy, Clone, PartialEq, Eq, Hash)]
@@ -230,6 +231,7 @@ mod test {
     use loop_analysis::{Loop, LoopAnalysis};
     use flowgraph::ControlFlowGraph;
     use dominator_tree::DominatorTree;
+    use std::vec::Vec;
 
     #[test]
     fn nested_loops_detection() {