Add some and, or, lt, gt, neq etc. helpers

[janbrasna]

It's easy to do #if a && b by doing two ifs… but it's super hard to do the else inversion when you only need to do it #unless a || b etc. … Leads to scary repetition, and the helpers are super easy one liners (even though their nested use get a bit scary sometimes, but… well… 🚀?)

[janbrasna]

(See e.g. 569144c for a few handy ones…)