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

examples/hello.rs

@@ -20,7 +20,7 @@ fn main() -> Result<()> {
     // Here we handle the imports of the module, which in this case is our
     // `HelloCallback` type and its associated implementation of `Callback.
     println!("Creating callback...");
-    let hello_func = Func::wrap0(&store, || {
+    let hello_func = Func::wrap(&store, || {
         println!("Calling back...");
         println!("> Hello World!");
     });