Don't require Store in Instance constructor (#810)

* Don't require `Store` in `Instance` constructor

This can be inferred from the `Module` argument. Additionally add a
`store` accessor to an `Instance` in case it's needed to instantiate
another `Module`.

cc #708

* Update more constructors

* Fix a doctest

* Don't ignore store in `wasm_instance_new`

* Run rustfmt

crates/api/src/runtime.rs

@@ -397,6 +397,10 @@ impl Store {
             .get(&type_index)
             .cloned()
     }
+
+    pub(crate) fn ptr_eq(a: &Store, b: &Store) -> bool {
+        Rc::ptr_eq(&a.inner, &b.inner)
+    }
 }
 
 impl Default for Store {