Use aliases for break types

[stackmystack]

When writing the Wadler interface, I noticed many calls like:

::Oppen::Token::BreakType::CONSISTENT

We should refactor and use aliases like:

CONSISTENT = ::Oppen::Token::BreakType::CONSISTENT

Should these constants be also exposes simply as Oppen::CONSISTENT ? I think the Oppen vocab is so small, that we can afford doing that, and may be a better solution that could benefit both Wadler and Oppen users.