feat(fun/🕰️): get the time as an emoji clock face

This small tool prints the current time rounded to half-hour precision
as an emoji clock face and exists. It can use both the local time zone
and UTC. Additionally it supports a pseudo dot time format.

Via fun.🕰️.lib we reexpose the internal library which allows conversion
from LOCAL-TIME:TIMESTAMP to an emoji clock face — maybe we'll want to
integrate this into //web/panettone?

//fun/🕰️ is the spritual (and actual) successor to
<https://github.com/sternenseemann/unicode_clock>.

It likely only works in SBCL due to its heavy usage of unicode symbol
names.

Change-Id: I44204107a14f99b04b0c5290d88e8659f013f423
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3164
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

fun/🕰️/lib.lisp

@@ -0,0 +1,32 @@
+(defpackage 🕰️
+  (:use :cl)
+  (:import-from :local-time
+   :timestamp-subtimezone :*default-timezone* :sec-of)
+  (:export :⌚))
+
+(in-package :🕰️)
+(declaim (optimize (safety 3)))
+
+(defparameter *clock-emojis*
+  (vector #\🕛 #\🕧   ; 00:00 - 00:30
+          #\🕐 #\🕜   ; 01:00 - 01:30
+          #\🕑 #\🕝   ; 00:00 - 00:30
+          #\🕒 #\🕞   ; 00:00 - 00:30
+          #\🕓 #\🕟   ; 00:00 - 00:30
+          #\🕔 #\🕠   ; 00:00 - 00:30
+          #\🕕 #\🕡   ; 00:00 - 00:30
+          #\🕖 #\🕢   ; 00:00 - 00:30
+          #\🕗 #\🕣   ; 00:00 - 00:30
+          #\🕘 #\🕤   ; 00:00 - 00:30
+          #\🕙 #\🕥   ; 00:00 - 00:30
+          #\🕚 #\🕦)) ; 11:00 - 11:30
+
+(defun ⌚ (timestamp &optional (tz *default-timezone*))
+  "Convert a LOCAL-TIME:TIMESTAMP into the nearest Unicode clock face.
+  Use TZ (which defaults to LOCAL-TIME:*DEFAULT-TIMEZONE*) to determine
+  the UTC offset to factor when determining the local clock face."
+  (let* ((offset (multiple-value-bind (offset-secs _dst _name)
+                   (timestamp-subtimezone timestamp tz)
+                   offset-secs))
+         (secs (+ (sec-of timestamp) offset)))
+    (elt *clock-emojis* (mod (round (/ secs 1800)) 24))))