Rewrite RevPrefixMap to avoid depending on internal details of PrefixMap

[mdickinson]

The RevPrefixMap trait type subclasses PrefixMap and depends on its internals. PrefixMap wasn't designed for subclassing, and indeed changes in Traits 6.3 broke RevPrefixMap as a result. I'd recommend rewriting RevPrefixMap as a direct subclass of TraitType.

[mdickinson]

Temporary workaround in Traits: enthought/traits#1578.

[mdickinson]

@rahulporuri Could we aim to include a fix for this in the Mayavi 4.7.4 release, too? That would give Traits the freedom to remove its workaround.

The fix I'd propose would be to start by simply pasting the Traits 6.2.0 version of PrefixMap directly into tvtk_base, and then incrementally rewriting to collapse the RevPrefixMap -> PrefixMap -> TraitType inheritance chain to just RevPrefixMap -> TraitType. We'd need some basic unit tests for RevPrefixMap, of course.

[rahulporuri]

@mdickinson makes sense. added this to the 4.7.4 milestone for now.