Rewrite Mutable_properties based on vector<SCM> (issue 575990043 by address@hidden)

[jonas . hahnfeld]

I gave this patch a try on my system, time lilypond MSDM.ly, 'real'
time, 3 repetitions

baseline (master, 0e7c26d40f):
0m42,533s; 0m42,547s; 0m42,430s

Abstract Grob property storage into Mutable_properties.
(https://codereview.appspot.com/561640043)
0m41,102s; 0m41,036s; 0m40,861s

Rewrite Mutable_properties based on vector<SCM>
(https://codereview.appspot.com/575990043/)
0m42,461s; 0m42,632s; 0m42,212s

(I did not apply any of the other optimization patches posted in the
last days.)
If at all, Issue 561640043 seems to show some benefit. This _could_ be
because Grob::Grob (Grob const &s) doesn't call ly_deep_copy anymore,
but I've only looked into this very briefly because I think the
introduction of Mutable_properties makes sense in any case. However I
clearly don't see 7% improvement for this patch. Is there anything I
missed?

https://codereview.appspot.com/575990043/