Preprocessor: Support creating empty arrays with [] (wishlist)

It would be great if [] returned an empty array. (At the moment it produces an error.) Doing [ "" ] - [ "" ] is a bit cumbersome!

@tholden Essentially this has not been implemented because the macroprocessor cannot determine the type of the empty array. I.e. it does not know if [] is an empty array of integers or of strings. The solution is probably to create a syntax to distinguish between these two types of arrays.

@tholden Empty arrays are now supported (starting from tomorrow's unstable snapshot).

Great, thanks.