Clean up Model::$delegate

[shmax]

The Model::$delegate feature is either a little half-baked and missing tests, or so simple that they're not needed, but either way it seems to be using the array<mixed> type which raises difficulties for static analysis. It's also complicated by the inclusion of a processed property that is just written right into the middle of the list, necessitating lots of careful runtime checking.

TODO:

• see if we can separate out processed
• convert to a array<string, DelegateOptions> format, similar to what you see with things like Model::$has_many.

[shmax]

@ipundit I need to take the rest of the evening to work on another project, but I think this is one of the last big TODO items we need to sort out before moving on to new features, if you're interested in taking a crack at it.

[shmax]

Never mind, I took care of it:
#52