Add the branching arrows -<, -<<, --<, and -as-<. (resubmitted)

[malaggan]

Note: I had to resubmit a new pull request instead of #279 because the previous pull request cannot be reopened.

Add parallel-threading macros inspired by https://github.com/rplevy/swiss-arrows. The general idea is the to complement the existing threading macros, which apply forms in sequence, to provide similar macros that apply forms in parallel.

For example: (-< 3 list 1+ 1- (* 2)) will produce (4 2 6), and (-< 3 + 1+ 1- (* 2)) will produce 12. The syntax is similar to the sequential threading macros (->, ->>, etc.) except that an extra parameter is needed: the "collector" that will take the forms as arguments. This choice was made, instead of returning a list, to allow for short-circuiting special forms like and to be used.

The use case I've personally encountered that motivated me to implement this was the need to pass the same value to several predicates and combine these predicates using and:

(and (listp element)
        (-> element length (>= 2))
        (-> element car atom)
        (-> element cadr alistp)
       element)

could be written as:

(-< element and
      listp
      (-> length (>= 2))
      (-> car atom)
      (-> cadr alistp)
      identity))

[alphapapa]

See #228.

[eshrh]

Is there any update on getting this merged? I like these macros a lot, but I have to include them in my config manually...

Honestly, though, I'm not a huge fan of including the "collector" as the second argument. I think it's better handled by a normal reduction, like in the implementations given by alphapapa which match swiss-arrows.