Weeder should consider RULES

[phadej]

E.g. in lens

{-# RULES
"bytes -> map"    traversedStrictTree = sets B.map        :: ASetter' B.ByteString Word8;
"bytes -> imap"   traversedStrictTree = isets imapB       :: AnIndexedSetter' Int B.ByteString Word8;
"bytes -> foldr"  traversedStrictTree = foldring B.foldr  :: Getting (Endo r) B.ByteString Word8;
"bytes -> ifoldr" traversedStrictTree = ifoldring ifoldrB :: IndexedGetting Int (Endo r) B.ByteString Word8;
 #-}

but weeder thinks ifoldrB is unused.