Change spanner-id to be a key instead of a string

Change spanner-id to be a key instead of a string

Also changes argument \= to be a key.


This has been taken out from the ongoing work of Nathan Chou as a
separate issue since it is a separate problem and needed a bit of
polish.  There are no convert-ly rules since stuff like \=3 or
\="word" will still work due to type coercion.  However, if spanner-id
has been tweaked or overridden (possible as of 2.16), the type will
now be wrong.  It is rather hard to find good convert-ly rules for
those so this issue/patch currently does not try.  I might try
figuring out some rules here (possibly based on pre-\= regression
tests/code) in a separate issue but they are not likely to be overly
reliable.

Backport-by: David Kastrup <dak@gnu.org>


Also contains:

Changes entries for \= and in-chord slurs