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/wasm/src/code_translator.rs

@@ -34,6 +34,7 @@ use state::{TranslationState, ControlStackFrame};
 use std::collections::{HashMap, hash_map};
 use environ::{FuncEnvironment, GlobalValue};
 use std::{i32, u32};
+use std::vec::Vec;
 
 /// Translates wasm operators into Cretonne IL instructions. Returns `true` if it inserted
 /// a return.