formatMany is partial for no good reason

[Profpatsch]

We’ve had production issues because of:

postgresql-simple/src/Database/PostgreSQL/Simple.hs

Lines 174 to 176 in 6836ea6

	formatMany :: (ToRow q) => Connection -> Query -> [q] -> IO ByteString
	formatMany _ q [] = fmtError "no rows supplied" q []
	formatMany conn q@(Query template) qs = do

I think the function signature should be changed to:

formatMany :: (ToRow q) => Connection -> Query -> NonEmpty q -> IO ByteString

The breaking change is pretty much worth it, because anybody using this function that does not put a check (trivial to get wrong) will have a sleeping bug that blows up at the worst times.