NodePattern: Set optimization

[marcandre]

This builds on #110 to optimize NodePattern with unions of literals like (send nil? {:method :other_method})

> rake compile '{1 2 3}'
# before:
(1 === node || 2 === node || 3 === node)
# after:
::RuboCop::AST::NodePattern::Sets::SET_1_2_3 === node

And SET_1_2_3 is set to Set[1, 2, 3]

[marcandre]

Merged 🎉 . To be released tomorrow.