Better Wasmtime API for embedder (#287)

* Migrate wasm-rust-api code to wasmtime.

wasmtime-api/src/trap.rs

@@ -0,0 +1,19 @@
+#[derive(Fail, Debug)]
+#[fail(display = "Wasm trap")]
+pub struct Trap {
+    message: String,
+}
+
+impl Trap {
+    pub fn new(message: String) -> Trap {
+        Trap { message }
+    }
+
+    pub fn fake() -> Trap {
+        Trap::new("TODO trap".to_string())
+    }
+
+    pub fn message(&self) -> &str {
+        &self.message
+    }
+}