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/trampoline/func.rs

@@ -80,7 +80,7 @@ unsafe extern "C" fn stub_fn(vmctx: *mut VMContext, call_id: u32, values_vec: *m
         (args, signature.returns.len())
     };
 
-    let mut returns = vec![Val::default(); returns_len];
+    let mut returns = vec![Val::null(); returns_len];
     let func = &instance
         .host_state()
         .downcast_mut::<TrampolineState>()