[ new ] Agda.Utils.Bag for sets with duplicates.
[ new ] Null instance for Bag.
[ refactor ] Utils.List.uniqBy to uniqOn with Map implementation.
[ opt ] Small optimization in Utils.List.sorted.

inits is not needed here as zipWith disregards the tail of the longer list.
[ new ] Agda.Utils.List.duplicates.

Faster than nub (xs \\ nub xs).
[ debug ] Prettier printing of patterns in Coverage.
[ refactor ] Use duplicates, unlessNull in ConcreteToAbstract.
[ Errors ] Changed error message for non-linear patterns.

No longer talkes about `left hand side' as this would be confusing for
pattern synonyms (where the pattern is on the right hand side).
[ Issue 1295 ] Fixed by checking linearity of pattern synonym.
[ Issue 1295 ] Test case.