Issue 5695: reduce dynamic casting

https://sourceforge.net/p/testlilyissues/issues/5695/

Here are a few miscellaneous things that I've been holding onto for a while because they would have been out of context in other reviews.  Each file is in a separate commit, though they're squashed in this review.

1: defer dynamic_cast in Tie_column::add_tie
2: reduce unsmobbing in Spaceable_grob::get_spring
3: reduce unsmobbing in Paper_column::is_musical

[dak, 2020-01-25 17:37:11]

https://codereview.appspot.com/575530107/diff/581530050/lily/spaceable-grob.cc
File lily/spaceable-grob.cc (right):

https://codereview.appspot.com/575530107/diff/581530050/lily/spaceable-grob.c...
lily/spaceable-grob.cc:92: break;
That looks still too complicated.  Just do return *next_spring; here, no need to
break out of the loop.  Then you don't need the variable spring at all (since it
will be 0 if the loop exits regularly) and can hijack its name for the more
cumbersome next_spring.  And the final return can just be return Spring ();

Oh, and the programming_error can be called unconditionally then.

[Dan Eble, 2020-01-25 18:11:55]

simplify Spaceable_grob::get_spring