Patch: Make #{ ... #} wagonloads more useful

[David Kastrup]

Previously #{ ... #} created only sequential music.  This changes it to
accept everything an assignment accepts.  Incompatibility to before:
#{ #} returns a void music expression, #{ music #} does not create a
sequential music list but just a single music event,
only #{ music music ... #} works just the same as before.

<URL:http://codereview.appspot.com/4974046>

-- 
David Kastrup