Rethink dummy var.

[porcuquine]

#7 implements a dummy var with name "_" for zero-arg functions. That works fine for now, but if/when _ becomes a legal symbol character (which it probably should), it will be possible to create bogus programs depending on the sugar. Like ((lambda () (+ _ 1)) 1).

This needs more thought if we stay with the dummy var approach.

[namin]

Note that we can always use a special character that is not allowed in the syntax. I picked _ in part because it's not allowed for now.

[porcuquine]

Yep, it's good.

I just made this note as a reminder to make sure we keep it true in the future. Also, some theories of symbols don't include any disallowed characters.

[arthurpaulino]

This issue will be closed once the function application optimization lands, which gets rid of the dummy var altogether