create Duration also through factory, allow token=None in factory
handle \\override and \\revert
make \simultaneous and \sequential keywords and a class on their own
handle assignments on toplevel and inside header, paper, layout
if \with is found in a context-specced music, read another argument.
handle \clef
also dispatch on token class when reading items
one dispatcher is enough
Source: dont yield a newline after the last block
make dispatch stuff much simpler
cache class lookup in reader
combine some stuff
dispatch markup and scheme too
centralise all dispatching functionality
handle voice separator, bar check, ties, slurs, articulations, fingering, string numbers and dynamics
simplify and improve scheme parsing
nice repr for Item
handle tag commands
handle \tweak
remove all the ly.lex prefixes