Only downside/hesitation is that this info *must* be collected at the callsite,
which means that it affects the performance of *all* created signals.
Adds ~30-50 nsecs per signal.
Previously:
It was possible to provide a vector of middleware fns when creating
signals or adding handlers, e.g.:
(event! ::ev-id1 {:middleware [fn1 fn2 ...]}),
(add-handler! ::handler-id1 <handler-fn> {:middleware [fn1 fn2 ...]})
After this commit:
Middleware is always expected to be a single fn, or nil.
A `comp-middleware` util has been added to make it easy to compose multiple
middleware fns into one.
Motivation:
The previous (auto-composition) behaviour was nice when adding handlers,
but incurred a (small but non-trivial) runtime cost when creating signals.
The actual benefit (convenience) of auto-composition is very small, so
I've decided to just remove this feature and add the `comp-middleware` util.
Note that I ruled out the option of doing auto-comp only when adding handlers
since that've meant an inconsistency and so complexity for little benefit.
