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.

misc/wasmtime-py/src/lib.rs

@@ -7,7 +7,7 @@ use crate::import::into_instance_from_obj;
 use crate::instance::Instance;
 use crate::memory::Memory;
 use crate::module::Module;
-use std::cell::RefCell;
+use core::cell::RefCell;
 use std::rc::Rc;
 use wasmtime_interface_types::ModuleData;