Differentiate order in Expression equality function

[kitchoi]

This PR makes trait("a") | trait("b") to be considered as NOT equivalent to trait("b") | trait("a"). Strictly speaking, this is true because the given change handler will be registered to "a" first and then to "b" when observe(handler, trait("a") | trait("b")) is called. When observe(handler, trait("b") | trait("a")) is called, the change handler will be registered to "b" first and then to "a".

The __eq__ is not used by the internal machinery, it is there to help users learning the expression. Internal observe machinery uses _as_graphs instead. So if the users want to see these two being considered equal, we can change this back without affecting anything.

Checklist

• Tests
• Update API reference (docs/source/traits_api_reference): autodoc handled this.
• Update User manual (docs/source/traits_user_manual): Does not exist yet.
• Update type annotation hints in traits-stubs

[mdickinson]

LGTM