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/legalizer/boundary.rs

@@ -25,6 +25,7 @@ use ir::{Function, DataFlowGraph, Inst, InstBuilder, Ebb, Type, Value, Signature
 use ir::instructions::CallInfo;
 use isa::TargetIsa;
 use legalizer::split::{isplit, vsplit};
+use std::vec::Vec;
 
 /// Legalize all the function signatures in `func`.
 ///