add richer operators for testing interval membership
Generalize symbolic set operators a bit.

Previously they were defined in Interval.scala but lived in the
spire.optiona.unicode package. Now, the implicit class has been
moved there, and a companion class for Set[A] has been added as
well.
very minor GUIDE typo
Fix typos, Remove unused imports
Merge pull request #271 from dusank/topic/Fix-typos-and-imports

Fix typos, Remove unused imports
Merge pull request #268 from non/topic/more-unicode

Topic/more unicode
Working changes.
Merge commit '356ce2cae66dd052aa85fedfe2eca5c8e149e909' into topic/interval-point
isSupersetOf can fail on empty intervals.