Make more code work with no_std. (#407)

* Make more code work with no_std.

no_std support is still incomplete, but this patch takes care of the
bulk of the straightforward parts.

wasmtime-environ/src/module_environ.rs

@@ -1,6 +1,9 @@
 use crate::func_environ::FuncEnvironment;
 use crate::module::{Export, MemoryPlan, Module, TableElements, TablePlan};
 use crate::tunables::Tunables;
+use alloc::boxed::Box;
+use alloc::string::String;
+use alloc::vec::Vec;
 use core::convert::TryFrom;
 use cranelift_codegen::ir;
 use cranelift_codegen::ir::{AbiParam, ArgumentPurpose};
@@ -10,9 +13,6 @@ use cranelift_wasm::{
     self, translate_module, DefinedFuncIndex, FuncIndex, Global, GlobalIndex, Memory, MemoryIndex,
     SignatureIndex, Table, TableIndex, WasmResult,
 };
-use std::boxed::Box;
-use std::string::String;
-use std::vec::Vec;
 
 /// Contains function data: byte code and its offset in the module.
 #[derive(Hash)]