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/traps.rs

@@ -29,7 +29,7 @@ fn test_trap_return() -> Result<(), String> {
     let hello_func = Func::new(&store, hello_type, Rc::new(HelloCallback));
 
     let imports = vec![hello_func.into()];
-    let instance = Instance::new(&store, &module, &imports)
+    let instance = Instance::new(&module, &imports)
         .map_err(|e| format!("failed to instantiate module: {:?}", e))?;
     let run_func = instance.exports()[0]
         .func()