Using a dedicated enum for the Try protocol

[udoprog]

While looking over #587 I realized that I'm not a huge fan of the current return value for the TRY protocol.

To summarize, since the introduction of the TRY protocol (#544) it uses Result to indicate whether the short circuit or not. While this meshes well with the existing type system (Value::Result already exists and does basically what we want it to) it lacks clarity and becomes a bit messy with types such as Result<T, Result<(), Error>>.

Here I'm proposing we adopt the ControlFlow enum for this protocol.

Internally it also means that we should be able to represent it as a Value to the degree that at least the internal Vm operations are supported, which basically means conversion to and from ControlFlow<Value, Value>.

CC: @ModProg

[ModProg]

Makes sense, I used Result because it worked and already existed. I guess there is a reason std uses ControlFlow.

[udoprog]

To get an idea of how difficult it is to add support for an external enum, check out #591 where I've added Value::Ordering.