Add optional layout syntax for match expression

[momohatt]

match [1, 2, 3] as list integer with { #1 :: _ -> True; #2 :: _ -> False }

This can be a little tricky as the starting symbol collides with the sequential pattern.

[egisatoshi]

If we support this optional layout syntax, I think we can remove the option to omit | when a pattern-match expression has only one match clause.
It will make the implementation of the parser simple.

[momohatt]

I agree that it'll make the parser simpler, but I'm not sure if it's worthwhile to add this optional layout syntax while sacrificing that feature... Maybe we need to reconsider adding this syntax, and it depends on how often we need to write multiple match clauses in single line.