Add \musicLength <music> function

\musicLength <music> returns the total duration of music as a moment.

convert-ly converts certain calls to #(ly:make-moment) with literal numbers into calls to \musicLength <duration>.

Automatic and manual documentation changes are in separate commits.

There are two remaining mentions of ly:make-moment in the English docs: in snippet descriptions. I found that my development environment is missing some perl modules required by makelsr and I do not have the energy to hunt them down.