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/tests/import_calling_export.rs

@@ -47,7 +47,7 @@ fn test_import_calling_export() {
 
     let imports = vec![callback_func.into()];
     let instance =
-        Instance::new(&store, &module, imports.as_slice()).expect("failed to instantiate module");
+        Instance::new(&module, imports.as_slice()).expect("failed to instantiate module");
 
     let exports = instance.exports();
     assert!(!exports.is_empty());