add keyword arguments to reader function literals

[AlexKnauth]

add keyword arguments to reader function literals and also allow an arbitrary number of arguments, and also handle skipped arguments, and both % and %1.
Examples:

#lang rackjure
(map #λ(+ % 1) '(1 2 3))                   ;=> '(2 3 4)
(map #λ(+ % %2) '(1 2 3) '(1 2 3))         ;=> '(2 4 6)
(#λ(apply list* % %&) 1 '(2 3))            ;=> '(1 2 3)
(#λ(* 1/2 %#:m (* %#:v %#:v)) #:m 2 #:v 1) ;=> 1        ; keyword-arguments
(define x (#λ"I am x"))
(#λx)                                      ;=> "I am x" ;the body doesn't have to be in parens
(#λ(begin (set! % "%") %1) "%1")           ;=> "%"      ;% means exactly the same as %1,
                                                        ;and you can even use both at the same time,
                                                        ;and even set!-ing one set!s the other.
(#λ%2 "ignored" "%2")                      ;=> "%2"     ;handles skipped arguments
(apply #λ%42 (build-list 42 add1))         ;=> 42       ;handles an arbitrary number of arguments

[greghendershott]

Very cool! Thanks!

It's nice that you extended it for arbitrary arguments. I like how you made % an alias for %1 using a rename transformer; great idea. And I really like that you extended it for #:keyword arguments, which hadn't occurred to me, and is awesome.

I'm going to update the Scribble docs, and reformat/refactor some of the code, then merge.