Add a first-class way of accessing caller's exports (#1290)

* Add a first-class way of accessing caller's exports

This commit is a continuation of #1237 and updates the API of `Func` to
allow defining host functions which have easy access to a caller's
memory in particular. The new APIs look like so:

* The `Func::wrap*` family of functions was condensed into one
  `Func::wrap` function.
* The ABI layer of conversions in `WasmTy` were removed
* An optional `Caller<'_>` argument can be at the front of all
  host-defined functions now.

The old way the wasi bindings looked up memory has been removed and is
now replaced with the `Caller` type. The `Caller` type has a
`get_export` method on it which allows looking up a caller's export by
name, allowing you to get access to the caller's memory easily, and even
during instantiation.

* Add a temporary note

* Move some docs

docs/embed-rust.md

@@ -149,12 +149,12 @@ looks like this:
 # (import "" "double" (func $double (param i32) (result i32))))"#)?;
 // First we can create our `log` function, which will simply print out the
 // parameter it receives.
-let log = Func::wrap1(&store, |param: i32| {
+let log = Func::wrap(&store, |param: i32| {
     println!("log: {}", param);
 });
 
 // Next we can create our double function which doubles the input it receives.
-let double = Func::wrap1(&store, |param: i32| param * 2);
+let double = Func::wrap(&store, |param: i32| param * 2);
 
 // When instantiating the module we now need to provide the imports to the
 // instantiation process. This is the second slice argument, where each