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/src/lib.rs

@@ -24,7 +24,7 @@ mod values;
 pub use crate::callable::Callable;
 pub use crate::externals::*;
 pub use crate::frame_info::FrameInfo;
-pub use crate::func::{Func, WasmRet, WasmTy};
+pub use crate::func::*;
 pub use crate::instance::Instance;
 pub use crate::module::Module;
 pub use crate::r#ref::{AnyRef, HostInfo, HostRef};