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/src/callable.rs

@@ -59,7 +59,6 @@ use wasmtime_runtime::Export;
 ///
 /// // Create module instance that imports our function
 /// let instance = wasmtime::Instance::new(
-///     &store,
 ///     &module,
 ///     &[times_two_function.into()]
 /// )?;