[CM] s7/Snd hooks

[Bill Schottstaedt]

I'd like to change the way hooks work in s7/Snd.  Here's a
description:

    https://ccrma.stanford.edu/software/snd/snd/s7.html#hooks

I would keep the old way working for *error-hook*, but otherwise
all the s7_hook functions would either go away or change
in some backwards-incompatible manner.  Would this be
a huge burden for anyone?

(It also means changing the scheme hook functions, but there's a
wrapper for that in s7.html).