Added a collect method that takes T => T functions.
Added overloaded collectInto methods to EquiSet and SortedEquiSet.
Started writing into, then backed out. Changed the EquiSet collectInto method that
takes a SortedEquiSet to return a SortedEquiSet.
Changed name of HashEquiSet to FastEquiSet and made it and TreeEquiSet
public again.
Put the TreeEquiSet toString back to TreeEquiSet instead of SortedEquiSet,
now that TreeEquiSet is public again.
Made FastEquiSet and TreeEquiSet constructors private[scalactic] to enable more efficient
transformations between EquiSet's and SortedEquiSet's.