Issue 3582: Let make-music accept existing music or alists as source

This makes it possible to replace, say, a @samp{NoteEvent} in
@var{event} with a @samp{SkipEvent} (preserving all information) by
writing

(make-music 'SkipEvent event)
Use make-music with music arguments
Issue 3580: Replace unwarranted uses of map with for-each and other Scheme cleanups

Note that some uses of map actually relied on execution order (namely
expecting map to behave like map-in-order) and thus were not just
inelegant because of the side effect.  There are also some other
mostly trivial simplification of Scheme constructs.
Issue 3579: Make "Adding extra fingering with scheme" snippet do something useful

This snippet was never operative in 2.16 due to the EventChord changes
of issue 2240.  This updates it into using a few 2.16 features in
order to make things simpler while retaining the basic purpose.
Run scripts/auxiliar/makelsr.py
Issue 3573: Calculate wordwrapped/justified lines using the stencil stacking primitives

In that manner, the spacing decisions are sure to be correct.
Issue 1939: showLastLength with hidden tie error message

This just stops tie events from registering when skipTypesetting is
set to #t.