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/src/trampoline/mod.rs

@@ -8,8 +8,8 @@ mod memory;
 mod table;
 
 use crate::r#ref::HostRef;
+use alloc::rc::Rc;
 use failure::Error;
-use std::rc::Rc;
 
 use self::func::create_handle_with_function;
 use self::global::create_global;