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

crates/api/tests/traps.rs

@@ -278,7 +278,7 @@ fn rust_panic_import() -> Result<()> {
         &module,
         &[
             func.into(),
-            Func::wrap0(&store, || panic!("this is another panic")).into(),
+            Func::wrap(&store, || panic!("this is another panic")).into(),
         ],
     )?;
     let func = instance.exports()[0].func().unwrap().clone();
@@ -329,7 +329,7 @@ fn rust_panic_start_function() -> Result<()> {
     .unwrap_err();
     assert_eq!(err.downcast_ref::<&'static str>(), Some(&"this is a panic"));
 
-    let func = Func::wrap0(&store, || panic!("this is another panic"));
+    let func = Func::wrap(&store, || panic!("this is another panic"));
     let err = panic::catch_unwind(AssertUnwindSafe(|| {
         drop(Instance::new(&module, &[func.into()]));
     }))