Deprecate the Symbol trait type

[mdickinson]

This PR deprecates the Symbol trait type, which was untested, unused in code that I have access to (with one exception, where it can be easily replaced), and has some slightly dangerous eval-based semantics. It's also not really a good fit for a trait, since it involves neither validation nor notification.

• Tests
• Update API reference (docs/source/traits_api_reference)
• [ ] Update User manual (docs/source/traits_user_manual) N/A
• [ ] Update type annotation hints in traits-stubs N/A

[corranwebster]

A fair bit of discussion out of band, but every use case we could think of wass better done other ways, plus this is not used anywhere that we could find.

This looks good to go.