Encoding fixes for Guile 2.x
Write header files and Festival XML as UTF-8
Introduce new function ly:gulp-file-utf8
and use it for song-basic-nonenglish.ly
to correctly gulp the written
Festival XML, fixing the last major difference when checking a test-baseline
from Guile 1.8 with LilyPond built against Guile 2.2.
(We cannot re-purpose `ly:gulp-file because it is also used to gulp binary files in the EPS backend.)