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/misc/py/src/lib.rs

@@ -120,7 +120,7 @@ pub fn instantiate(
         }
     }
 
-    let instance = wasmtime::Instance::new(&store, &module, &imports)
+    let instance = wasmtime::Instance::new(&module, &imports)
         .map_err(|t| PyErr::new::<Exception, _>(format!("instantiated with trap {:?}", t)))?;
 
     let module = Py::new(py, Module { module })?;