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/examples/gcd.rs

@@ -50,7 +50,7 @@ fn main() -> anyhow::Result<()> {
         .0;
 
     // Instantiate the module.
-    let instance = Instance::new(&store, &module, &[])?;
+    let instance = Instance::new(&module, &[])?;
 
     // Invoke `gcd` export
     let gcd = instance.exports()[gcd_index].func().expect("gcd");