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/state.rs

@@ -7,6 +7,7 @@ use cretonne::ir::{self, Ebb, Inst, Value};
 use environ::{FuncEnvironment, GlobalValue};
 use std::collections::HashMap;
 use translation_utils::{GlobalIndex, MemoryIndex, SignatureIndex, FunctionIndex};
+use std::vec::Vec;
 
 /// A control stack frame can be an `if`, a `block` or a `loop`, each one having the following
 /// fields: