Implement 'wrapper' enum types

[Antikyth]

E.g. the CreateWindow request has the field visual, where 0 means CopyFromParent, and any other value means a VisualId. This will need to therefore be something like:

pub enum Inheritable<T> {
    CopyFromParent,
    Uninherited(T),
}

where Uninherited(T) writes no discriminant, and CopyFromParent, in the case of VisualIds, is written as a u32 value (as are VisualIds).

Thoughts

May be able to take advantage of the fact that VisualIds actually wrap u32 values. Traits could be implemented for Inheritable<T> where T wraps a u32 value (what bound that could be, I'm not sure; T: From<u32> wouldn't work because of the fact that it would be possible to implement From<u16>, etc., for T as well).

[Antikyth]

This is one of the most challenging unsolved problems in XRB: how to do this in a decent way.

[Antikyth]

Currently this is being done with a Wrapper trait in XRBK. There may or may not be a better way. I will leave this issue open for now in case a better way is found.

[Antikyth]

See #45 and #53 draft PRs.

[Antikyth]

Completed in #69.