emacs: make subr-x available in all libraries

Like `cl-lib' and `pcase', which are already available in all
libraries, `subr-x' also provided many useful functions that
we would like to use.

Making `subr-x' available in every library from the get-go means
that we can use the functions it defines without having to double
check every single time, whether the feature is already available
in the current library.