no argument function syntax

there doesn't appear to be a way to spell functions with no arguments, such as something like `current_timestamp` in some sql databases.

I tried `() -> datetime` and `(void) -> datetime`, and `(null) -> datetime` but none are valid datashapes. @mwiebe and @izaid might have a way to do this in libdynd. We should adopt their syntax if so.

cc @llllllllll