After some profiling, it turns out map-put and map-delete are 5-7x
slower (more on Emacs 25) than delq, setf/alist-get and add-to-list for
small lists (under 250 items), which is exactly how I've been using
them.
The only caveat is alist-get's signature is different on Emacs 25, thus
a polyfill is necessary in core-lib.
f6dc6ac74e