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/module_translator.rs

@@ -8,6 +8,8 @@ use sections_translator::{SectionParsingError, parse_function_signatures, parse_
                           parse_elements_section, parse_data_section};
 use environ::ModuleEnvironment;
 
+use std::string::String;
+
 /// Translate a sequence of bytes forming a valid Wasm binary into a list of valid Cretonne IL
 /// [`Function`](../cretonne/ir/function/struct.Function.html).
 /// Returns the functions and also the mappings for imported functions and signature between the