Implement $ operator, second class version of the applicative operator

The `$` operator takes a sequence of FORMS and returns a unary
function which applies the input through that sequence via the `->>`
operator.

For example, consider the predicate "not null".  `null` is built into
Common Lisp but "not null" requires writing a
function (lambda (x) (not (null x))).  Now, using this operator, you
can write ($ not null) which returns the same lambda as above while
being more concise.

lib.macros.lisp

@@ -77,3 +77,9 @@ arguments `LAMBDA-LIST' with body `BODY'."
      (declaim (ftype ,type ,name))
      (defun ,name ,lambda-list
        ,@body)))
+
+(defmacro $ (&rest forms)
+  "Given a sequence of FORMS, return a unary function which applies each form
+sequentially"
+  `(lambda (x)
+     (->> x ,@forms)))

packages.lisp

@@ -22,7 +22,8 @@
   (:export
    :_ :--> :->>
    :while
-   :-> :fn))
+   :-> :fn
+   :$))
 
 (defpackage lib.functions
   (:use :cl :lib.macros)