Better Wasmtime API for embedder (#287)

* Migrate wasm-rust-api code to wasmtime.

wasmtime-api/src/trampoline/mod.rs

@@ -0,0 +1,19 @@
+//! Utility module to create trampolines in/out WebAssembly module.
+
+mod code_memory;
+mod create_handle;
+
+use failure::Error;
+use std::cell::RefCell;
+use std::rc::Rc;
+
+use self::create_handle::create_handle;
+use super::externals::Func;
+
+pub fn generate_func_export(f: &Rc<RefCell<Func>>) -> Result<(), Error> {
+    let mut instance = create_handle(f)?;
+    let export = instance.lookup("trampoline").expect("trampoline export");
+
+    f.borrow_mut().anchor = Some((instance, export));
+    Ok(())
+}