Implement RFC 11: Redesigning Wasmtime's APIs (#2897)

Implement Wasmtime's new API as designed by RFC 11. This is quite a large commit which has had lots of discussion externally, so for more information it's best to read the RFC thread and the PR thread.

tests/all/store.rs

@@ -0,0 +1,22 @@
+use std::sync::atomic::{AtomicUsize, Ordering::SeqCst};
+use wasmtime::{Engine, Store};
+
+#[test]
+fn into_inner() {
+    static HITS: AtomicUsize = AtomicUsize::new(0);
+
+    struct A;
+
+    impl Drop for A {
+        fn drop(&mut self) {
+            HITS.fetch_add(1, SeqCst);
+        }
+    }
+
+    let engine = Engine::default();
+    assert_eq!(HITS.load(SeqCst), 0);
+    drop(Store::new(&engine, A));
+    assert_eq!(HITS.load(SeqCst), 1);
+    Store::new(&engine, A).into_data();
+    assert_eq!(HITS.load(SeqCst), 2);
+}