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/examples/memory.rs

@@ -51,7 +51,7 @@ macro_rules! call {
   ($func:expr, $($p:expr),*) => {
     match $func.borrow().call(&[$($p.into()),*]) {
       Ok(result) => {
-        let result: i32 = result[0].clone().into();
+        let result: i32 = result[0].unwrap_i32();
         result
       }
       Err(_) => { bail!("> Error on result, expected return"); }