Add function `sepBy1Cont`

[lamg]

I found a case where it might be useful to add a parser combinator which returns two things:

• a value built with what is meant to recognize
• a parser or a value useful for continuing parsing after that string

This can happen when you have a clause in SQL like SELECT column0, … column_N (FROM | ORDER BY), where you might have a function for parsing the column list, and then different functions for parsing what comes after FROM or ORDER BY.

I explain that situation with more detail here

sepBy1Cont could be used the following way

type Either<'a, 'b> =
    | Left of 'a
    | Right of 'b

let sepCont =
    parse {
        do! symbol S.Comma
        return Left ()
    }
    <|> parse {
        do! keyword K.From
        return Right From
    }
    <|> parse {
        do! keyword K.Order
        do! keyword K.By
        return Right OrderBy
    }

let! xs, next = sepBy1Cont identifier sepCont