Liquid's boolean logic does not support not, so is not functionally complete #526
Description
Liquid should support not — not only because it is the obvious thing to do, but also because it is required to write arbitrary boolean conditions. unless is not sufficient for nested negations --- @nickpearson shows how to compile not by hand in this comment, and the amount of extra needed code is significant.
Moreover, the documentation claims:
Liquid has access to all of the logical and comparison operators.
but because of the absence of not, that's not true.
(I know that there is #138, but it was closed without sufficient discussion).