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/solver.rs

@@ -108,6 +108,7 @@ use std::fmt;
 use std::mem;
 use super::AllocatableSet;
 use std::u16;
+use std::vec::Vec;
 
 /// A variable in the constraint problem.
 ///
@@ -1162,6 +1163,7 @@ mod tests {
     use isa::{TargetIsa, RegClass, RegUnit, RegInfo};
     use regalloc::AllocatableSet;
     use super::{Solver, Move};
+    use std::boxed::Box;
 
     // Make an arm32 `TargetIsa`, if possible.
     fn arm32() -> Option<Box<TargetIsa>> {