refactor(3p/lisp/mime4cl): remove be and be*

Seems simple enough to use standard LET and a few parentheses more which
stock emacs can indent probably.

Change-Id: I0137a532186194f62f3a36f9bf05630af1afcdae
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8584
Reviewed-by: sterni <sternenseemann@systemli.org>
Autosubmit: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

third_party/lisp/mime4cl/streams.lisp

@@ -138,7 +138,7 @@ in a stream of character."))
   (with-slots (encoder buffer-queue real-stream) stream
     (loop
        while (queue-empty-p buffer-queue)
-       do (be byte (read-byte real-stream nil)
+       do (let ((byte (read-byte real-stream nil)))
             (if byte
                 (encoder-write-byte encoder byte)
                 (progn