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/sections_translator.rs

@@ -16,6 +16,8 @@ use wasmparser::{Parser, ParserState, FuncType, ImportSectionEntryType, External
 use wasmparser;
 use std::str::from_utf8;
 use environ::ModuleEnvironment;
+use std::vec::Vec;
+use std::string::String;
 
 pub enum SectionParsingError {
     WrongSectionContent(String),