#4954 Change spanner-id to be a key instead of a string

[David Kastrup]

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

http://codereview.appspot.com/306950043

[Anonymous]

Passes make, make check and a full make doc.

[Anonymous]

Patch on countdown for August 12th

[Anonymous]

Patch counted down - please push

[Anonymous]

Patch counted down - please push

[David Kastrup]

Pushed to staging as
commit 0bab8fec496686ed5d3a086085fa068c41974c42
Author: David Kastrup dak@gnu.org
Date: Sun Aug 7 13:57:25 2016 +0200

Issue 4954/2: Changes entries for \= and in-chord slurs

commit 12b68a3172f040e768b7d229aa14fb4838e75742
Author: Nathan Chou starrynte@gmail.com
Date: Mon Jul 25 17:45:29 2016 -0700

Issue 4954/1: Change spanner-id to be a key instead of a string

Also changes argument \= to be a key.

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