Work with #![no_std] contexts

This commit adds support for both `#![no_std]` in the wasm-bindgen runtime
support (disabled by default with an on-by-default `std` feature). This also
adds support to work and compile in the context of `#![no_std]` crates.

Closes #146

src/closure.rs

@@ -7,6 +7,7 @@
 use std::cell::UnsafeCell;
 use std::marker::Unsize;
 use std::mem::{self, ManuallyDrop};
+use std::prelude::v1::*;
 
 use JsValue;
 use convert::*;