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

src/commands/run.rs

@@ -259,7 +259,7 @@ impl RunCommand {
             })
             .collect::<Result<Vec<_>, _>>()?;
 
-        let instance = Instance::new(store, &module, &imports)
+        let instance = Instance::new(&module, &imports)
             .context(format!("failed to instantiate {:?}", path))?;
 
         Ok((instance, module, data))