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/wast/src/wast.rs

@@ -99,7 +99,7 @@ impl WastContext {
                 .clone();
             imports.push(export);
         }
-        let instance = match Instance::new(&self.store, &module, &imports) {
+        let instance = match Instance::new(&module, &imports) {
             Ok(i) => i,
             Err(e) => return e.downcast::<Trap>().map(Outcome::Trap),
         };