It's kind of silly that our command lambda macros (λ! and λ!!) need a
snippet, special key sequence or copy-paste to insert, so in the spirit
of fn! -- and to make sure they take up less space than `lambda!` --
I've added `cmd!` and `cmd!!` aliases. `lambda!` and `lambda!!` are now
deprecated. λ! and λ!! will remain.
I've also added `cmds!` as a convenience wrapper around
general-predicate-dispatch.