Support behavioral dependencies

[handrews]

Currently, keywords can only depend on specific other keywords. However, the specification (particularly the unevaluated* keywords) requires depending on in-place applicators as a behavioral class. If a custom in-place applicator is used, it is expected to automatically affect unevaluated*.

This requires marking the behavior and also sorting dependencies accordingly.

Behavior marking could be done with a base class or mixin, but given other architectural decisions in this project, it seems more appropriate to use class attributes similar ot the existing depends_on and instance_types attributes.

The only behavior that currently needs this support is in-place application. I would be disinclined to add further behaviors without use cases, but the mechanism should work for any future behavior should the need arise.