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.

lib/wasm/src/environ/dummy.rs

@@ -10,6 +10,8 @@ use cretonne::cursor::FuncCursor;
 use cretonne::settings;
 use wasmparser;
 use std::error::Error;
+use std::vec::Vec;
+use std::string::String;
 
 /// Compute a `ir::ExternalName` for a given wasm function index.
 fn get_func_name(func_index: FunctionIndex) -> ir::ExternalName {

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)]

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

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),

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: