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-api/examples/hello.rs

@@ -1,9 +1,11 @@
 //! Translation of hello example
 
+extern crate alloc;
+
+use alloc::rc::Rc;
+use core::cell::Ref;
 use failure::{bail, format_err, Error};
-use std::cell::Ref;
 use std::fs::read;
-use std::rc::Rc;
 use wasmtime_api::*;
 
 struct HelloCallback;