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/spilling.rs

@@ -26,6 +26,7 @@ use regalloc::liveness::Liveness;
 use regalloc::pressure::Pressure;
 use regalloc::virtregs::VirtRegs;
 use std::fmt;
+use std::vec::Vec;
 use timing;
 use topo_order::TopoOrder;