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/hello.rs

@@ -48,8 +48,7 @@ fn main() -> Result<()> {
     // Note that this is where the wasm `start` function, if any, would run.
     println!("Instantiating module...");
     let imports = vec![hello_func.into()];
-    let instance =
-        Instance::new(&store, &module, &imports).context("> Error instantiating module!")?;
+    let instance = Instance::new(&module, &imports).context("> Error instantiating module!")?;
 
     // Next we poke around a bit to extract the `run` function from the module.
     println!("Extracting export...");