Review deprecations for potential removal

[mdickinson]

This is a placeholder issue for Traits 7.0: a reminder to review the various pieces of deprecated functionality in the Traits codebase and consider removing them.

We'll need to make the decision on a case-by-case basis - some of the deprecated features have little evidence of wider usage and are fairly safe to remove; some may need to be treated more carefully, with removal possibly deferred until Traits 8.0 or later (for example the Unicode trait).

Note: on_trait_change is not currently deprecated, and will not be removed in Traits 7.0!

Here's a preliminary list of candidates for removal:

• [ ] The ability to call CTrait.default_value with a nonzero number of arguments.
• [ ] configure_traits support for restoration from a pickle file
• [ ] The ability of provides to perform interface checking when CHECK_INTERFACES is nonzero
• [ ] The typecode parameter to Array and CArray
• [ ] Support for rich_compare metadata on traits
• [ ] The traits.testing.nose_tools package
• [ ] _TraitsChangeCollector support for the trait parameter
• [ ] HasTraits.set and HasTraits.get
• [ ] SingletonHasTraits, SingletonHasStrictTraits and SingletonHasPrivateTraits
• [ ] MetaInterface implicit adaptation on instantiation
• [ ] TraitPrefixList, TraitMap, TraitPrefixMap
• [ ] TraitTuple, TraitList and TraitDict
• [ ] Color, RGBColor, Font
• [ ] The Long trait type
• [ ] The Unicode trait type (this one will likely have to live beyond Traits 7.0)
• [ ] AdaptedTo as an alias for Supports
• [ ] false, true, undefined trait type aliases
• [ ] ListInt, ListThis and other List* and Dict* trait types