Allow discards in LINQ

[dsaf]

It would be useful for some "non-query" internal DSLs such as parser combinators e.g. https://github.com/datalust/superpower.

For example:

private static readonly TokenListParser<MyToken, KeyValueNode> KeyValue =
    from _ in Token.EqualTo(MyToken.KeyValueKeyword)
    from _ in Token.EqualTo(MyToken.LeftParenthesis)
    from key in Text
    from _ in Token.EqualTo(MyToken.Comma)
    from value in Text
    from _ in Token.EqualTo(MyToken.RightParenthesis)
    select new KeyValueNode(key, value);

[GreemDev]

While nice, extension-LINQ can do this. I'm not opposed to this personally, but it just seems...un-necessary.