Tweak the API of the Val type (#679)

* Tweak the API of the `Val` type

A few updates to the API of the `Val` type:

* Added a payload for `V128`.
* Replace existing accessor methods with `Option`-returning versions.
* Add `unwrap_xxx` family of methods to extract a value and panic.
* Remove `Into` conversions which panic, since panicking in `From` or
  `Into` isn't idiomatic in Rust
* Add documentation to all methods/values/enums/etc.
* Rename `Val::default` to `Val::null`

* Run rustfmt

* Review comments

crates/api/src/wasm.rs

@@ -1553,7 +1553,11 @@ unsafe fn into_funcref(val: Val) -> *mut wasm_ref_t {
     if let Val::AnyRef(AnyRef::Null) = val {
         return ptr::null_mut();
     }
-    let r = Box::new(wasm_ref_t { r: val.into() });
+    let anyref = match val.anyref() {
+        Some(anyref) => anyref,
+        None => return ptr::null_mut(),
+    };
+    let r = Box::new(wasm_ref_t { r: anyref });
     Box::into_raw(r)
 }