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/interface-types/src/lib.rs

@@ -152,7 +152,14 @@ impl ModuleData {
             Ok(values) => values
                 .to_vec()
                 .into_iter()
-                .map(|v: wasmtime::Val| v.into())
+                .map(|v: wasmtime::Val| match v {
+                    wasmtime::Val::I32(i) => RuntimeValue::I32(i),
+                    wasmtime::Val::I64(i) => RuntimeValue::I64(i),
+                    wasmtime::Val::F32(i) => RuntimeValue::F32(i),
+                    wasmtime::Val::F64(i) => RuntimeValue::F64(i),
+                    wasmtime::Val::V128(i) => RuntimeValue::V128(i),
+                    _ => panic!("unsupported value {:?}", v),
+                })
                 .collect::<Vec<RuntimeValue>>(),
             Err(trap) => bail!("trapped: {:?}", trap),
         };