I'm not fully convinced with swapping out ido with selectrum, so I'll
add the configuration for both, without actually loading them, and
instead manually try them out from time to time.
Also, make ivy use prescient for filtering and sorting, as it does a
much better job than ivy itself does by default. But nothing currently
uses ivy either, so it's more for future if I start using ivy somewhere.
Smex has not been updated since late 2015. Amx was originally a fork of
smex, but as since evolved much further into it's own thing.
Among other things is supports other completing backends, and ivy with
it's fuzzy matching feels rather nice. I'm keeping ido-mode for
everything else, but specifically for M-x I'll give ivy a try.
