Better lilypond variable names

[noteflakes]

It turns out numbers and other symbols can be used in variable names, provided they are quoted.

So, instead of doing the whole dance of converting numbers to roman numbers, capitalizing, upcasing etc, we use a much simpler variable naming scheme, which reflects the stream path in the work context:

"01-choro/soprano/music" = { ... }
"01-choro/soprano/lyrics/..." = { ... }
"01-choro/violino1/music" = { ... }

% usage:
... \"01-choro/violino1/music" ...