[Proposal] Adding (back ?) chainl

Hi !

Is it possible to add the chainl, chainl1, chainr, chainr1 combinators ? Although Control.Monad.Combinators.Expr does exists, it feels overkill to use when there's only one layer to the chain; and I don't really understand why it was removed from megaparsec

The implementation would be pretty straightforward :

chainl1 parser op = parser >>= rest
  where rest x = do { f <- op; y <- parser; rest (f x y) } <|> return x

chainl parser op x = chainl1 parser op <|> return x

and so on...

Thanks in advance :) !

Start by reviewing the package's existing exports and parser-combinator conventions, then compare the requested chainl, chainl1, chainr, and chainr1 combinators with the implementation shown in the issue. Done means the requested combinators are added consistently to the package and their behavior is covered by the project's existing checks.