Issue 2671: Provide functions ly:moment-main and ly:moment-grace

Issue 2671: Provide functions ly:moment-main and ly:moment-grace

Those will usually be more convenient than using ly:moment-*-numerator
and ly:moment-*-denominator.

Also, (/ (ly:moment-*-numerator ...) (ly:moment-*-denominator ...))
can't deal with infinite moment components.  This could theoretically
be amended by letting (ly:moment-*-denominator ...) resolve to
_inexact_ zero (in which case the division turns out ±inf.0), but that
is all rather hand-wavy and flimsy.  Being able to access the
components directly is more straightforward.