Better Wasmtime API for embedder (#287)

* Migrate wasm-rust-api code to wasmtime.

wasmtime-api/examples/gcd.rs

@@ -0,0 +1,45 @@
+//! Example of instantiating of the WebAssembly module and
+//! invoking its exported function.
+
+use failure::{format_err, Error};
+use std::cell::RefCell;
+use std::fs::read;
+use std::rc::Rc;
+use wasmtime_api::*;
+
+fn main() -> Result<(), Error> {
+    let wasm = read("examples/gcd.wasm")?;
+
+    // Instantiate engine and store.
+    let engine = Rc::new(RefCell::new(Engine::default()));
+    let store = Rc::new(RefCell::new(Store::new(engine)));
+
+    // Load a module.
+    let module = Rc::new(RefCell::new(Module::new(store.clone(), &wasm)?));
+
+    // Find index of the `gcd` export.
+    let gcd_index = module
+        .borrow()
+        .exports()
+        .iter()
+        .enumerate()
+        .find(|(_, export)| export.name().to_string() == "gcd")
+        .unwrap()
+        .0;
+
+    // Instantiate the module.
+    let instance = Rc::new(RefCell::new(Instance::new(store.clone(), module, &[])?));
+
+    // Invoke `gcd` export
+    let gcd = instance.borrow().exports()[gcd_index]
+        .borrow()
+        .func()
+        .clone();
+    let result = gcd
+        .borrow()
+        .call(&[Val::from(6i32), Val::from(27i32)])
+        .map_err(|e| format_err!("call error: {:?}", e))?;
+
+    println!("{:?}", result);
+    Ok(())
+}