Hi Emacs developers,

Reading eval.c I realized that there is very similar code in both
'eval_sub' and 'funcall_subr', where they invoke the subroutine itself.

I figured, since we have 'apply_lambda' (that gets called from
'eval_sub'), why not have an 'apply_subr' as well, to be used for
subroutines? So I wrote a small patch (WIP) that adds 'apply_subr',
which in turn calls 'funcall_subr'. I had to adapt 'funcall_subr' so
that it accepts 'max_args=UNEVALLED' subroutines.

I think the advantages of doing this are that 1) it should make making
changes to the structure of subroutines slightly easier (less code to
update!) and 2) makes 'eval_sub' much more readable. In fact, now the
function-calling part of 'eval_sub' is relatively short (~45 lines),
which makes understanding the general structure of the function much
easier in my opinion.

My concerns now are:
1) Could I have broken anything without realizing it, since this is such
a central function in Lisp code evaluation? Everything seems to be
compiling fine (without warnings) and so far I haven't had any crashes.
2) I removed a comment that made reference to Bug#21245, but it seems
like it makes sense since the variable it refers to is no longer needed.
3) Have I maybe made Emacs slower by always using SAFE_ALLOCA_LISP for
the subroutine arguments (instead of only for 'max_args=MANY')?

Any feedback is appreciated, in order to decide if it makes sense to
work further on this.

Thanks!