`Optional` trait type

[k2bd]

Closes #1298

This PR:

• Creates the Optional trait type as a shorthand for Union(None, ...)
• Changes exception messages in Union to refer to subclass names, e.g. Optional (let me know if this is controversial and I can change!)
• Adds xfail tests for Constant, Union, and Optional demonstrating behaviour discussed in #1784, some of these may also be controversial and I can remove/alter them if so

Checklist

• [x] Tests
• [x] Update API reference (docs/source/traits_api_reference)
• [x] Update User manual (docs/source/traits_user_manual)
• [x] Update type annotation hints in stub files