Users caught by this change should receive a clear compile-time error.
Apologies for the nuissance!! This change is part of a final review
of names before the release of v1 final.
Motivations:
- "xfn" is a lot shorter than "middleware", making it more
convenient to use at signal calls, compare:
(log! {:middleware my-fn} "msg")
(log! {:xfn my-fn} "msg"}
- "middleware" was originally chosen to match Timbre's terminology,
but actually carries some misleading connotations that in hindsight
are probably better avoided while we still have the chance to change
this.
