Kast in k

I created this diff for use in managing the discussion on the KORE syntax. We may choose to close it rather than merge it, I just need a place to put comments.
Which one?  Parentheses and brackets are taken, almost all languages use them.  Backtick is typically available, and we only reserve one character now; hard to improve over that.  Brackets are going to be rarely used in general, anyway.  The analogy to markdown's "bubbles" is also nice and intuitive.