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/tests/import_calling_export.rs

@@ -1,6 +1,8 @@
-use std::cell::{Ref, RefCell};
+extern crate alloc;
+
+use alloc::rc::Rc;
+use core::cell::{Ref, RefCell};
 use std::fs::read;
-use std::rc::Rc;
 use wasmtime_api::*;
 
 #[test]