Segmenter cleanups, part 2 (reduce trait complexity)

[Manishearth]

First set of followups after #6409

This:

• Removes lifetimes and ?Sized from the BreakType traits
• Coalesces the BreakType markers into a single set of RuleBreakTypeFoo markers
• Makes LineBreakType depend on RuleBreakType
• Adds a new WordBreakType that depends on RuleBreakType

previously the wordbreaktype distinction was awkwardly handled by unreachable!()s and trait impls; which meant it was not straightforward to make WordBreak use RuleBreakType instead of WordBreakType. Moving that required moving it to a function, but it seemed to work well.

This does not intend to finalize the module structure, it's still doing cleanups, I didn't wish to have these be blocked on module structure decisions.

A cleanup I may make is to use a secondary type parameter on RuleBReakIterator instead of a function pointer. It ought not matter, though.