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/environ/spec.rs

@@ -5,6 +5,8 @@ use cretonne::cursor::FuncCursor;
 use cretonne::settings::Flags;
 use translation_utils::{SignatureIndex, FunctionIndex, TableIndex, GlobalIndex, MemoryIndex,
                         Global, Table, Memory};
+use std::vec::Vec;
+use std::string::String;
 
 /// The value of a WebAssembly global variable.
 #[derive(Clone, Copy)]