refactor!(lib): rename fn!->lambda! & fn!!->fn!

BREAKING CHANGE: This renames the fn! macro to lambda! and fn!! to fn!.
I hadn't put much thought into their names when they were added, but now
that they're seeing more use, I've reconsidered.

The reasoning is (and I'll refer to them by their new names):

- If you're using fn!, you care more about the syntax's brevity, than if
  you were using lambda!, so I wanted fn! to have the (even if slightly)
  shorter name.
- lambda! decorates native lambda (with cl-function). Its old name
  did not suggest that connection like other !-macros in Doom's library
  do.
- Their old names implied the two macros were somehow related or that
  one decorated the other. They aren't and don't.

core/autoload/print.el

@@ -321,9 +321,9 @@ Each argument in ARGS can be a list, as if they were arguments to `format!':
 
 `nil' and empty strings in SEQUENCE are omitted."
   (mapconcat (doom-partial #'format "%s")
-             (seq-remove (fn!! (or (null %)
-                                   (and (stringp %)
-                                        (string-empty-p %))))
+             (seq-remove (fn! (or (null %)
+                                  (and (stringp %)
+                                       (string-empty-p %))))
                          sequence)
              (or separator " ")))