instruments: simplify the function thanks to Davids work
instruments: pass parent and settings as parameters
instruments: pass parent context name as string argument (UGH)
instruments: use a context-mod instead of music

(this is the correct way - with ly:music? it was impossible to e.g. add engravers)
instruments: define Vocal using newInstrument
instruments: pass parent voice as another argument (UGH)
instruments: implement Soprano using newInstrument
instruments: change description and remove Test instrument

since Vocal and Soprano are implemented using newInstrument, we dont need a separate Test