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/frontend/src/ssa.rs

@@ -15,6 +15,7 @@ use std::u32;
 use cretonne::ir::types::{F32, F64};
 use cretonne::ir::immediates::{Ieee32, Ieee64};
 use std::mem;
+use std::vec::Vec;
 
 /// Structure containing the data relevant the construction of SSA for a given function.
 ///