Add alternative syntax for methods

[rpgoldman]

(Based on discussion with @gefjon)

The current syntax for methods is quite hard to read, especially when there are multiple if-then-else branches.

Suggest we create an alternative syntax that's easy to read for the simple case and introduce :if :elsif and :else keywords for processing the more complex branching structure.

[gefjon]

I would use :elif rather than :elsif. The extra letter is no more clear (since it's still not complete like :elseif or :else-if), and Python's use of elif has cemented the 4-letter version as standard.

[ko56]

Enhancing the method syntax would definitely be an improvement. But I would vote for :elseif, which is what Julia uses.

[ko56]

A proposal: if the work to implement a/the new syntax is not too much, how about implementing something so that people can play with it, with the understanding that it is subject to change?