EXPERIMENTAL: compiled vm callback

[schungx]

@ImTheSquid I have pushed an experimental branch called experimental-compiled-vm-callback which illustrates my idea. Can you take a look? For reference only at this point.

What it does:

• Eliminated a lot of the stuff such as makes_pointer, needs_this, etc.

• Eliminated the need to keep a lib of the AST for walking. This has the side effect that some features such as import, eval etc. won't compile.

• Seems to work with anything I've thrown at it so far...

• Eliminated the need to consider arity in callbacks... everything just works, because the arity is not in question -- the function is scripted, and it always has this (even if unused), so the shape is fixed.

• Surgical changes in fn_ptr.rs... It is now simply FnPtrType::Compiled with no attached info. Just a marker.

• There is no observed regression in performance.

The essence is these two changes. They make sure compiled chunks are always called with a this pointer at the front (it may be a dummy if not used). This takes care of all our arity problems.

#[cfg(all(feature = "grain", not(feature = "no_function")))]
// VM-compiled chunk
FnPtrType::Compiled => {
    // Since the chunk is actually a script-ed function, the `this` pointer goes into
    // the first position.
    let args = &mut arg_values.iter_mut().collect::<FnArgsVec<_>>();
    if let Some(this_ptr) = this_ptr.as_deref_mut() {
        args.insert(0, this_ptr);
    } else {
        args.insert(0, &mut dummy);
    }
    return context.call_native_fn_raw(self.fn_name(), true, args);
}

// A compiled chunk automatically takes an additional argument
// for `this`, push to the very front.
let arity = function.params.len() + 1;

                :

let wrapper = move |context: Option<NativeCallContext>, args: &mut [&mut Dynamic]| {
    // Always extracts the first argument for `this`.
    let (this_ptr, args) = args
        .split_first_mut()
        .ok_or_else(|| malformed("a callback wrapper was given no `this`".into()))?;

    invoke_with_this(&owner, &called, context.as_ref(), args, Some(this_ptr))
};