Ornaments (trills, appoggiaturas, etc.)

[daveyarwood]

Moved from #180.

From an anonymous comment on my blog post introducing Alda:

• decorators to realize ornaments like trills, mordents, turns, appogiaturas, acciaccaturas, glissandos, ...

Opening this issue as a starting point for discussion on how we can do this, or whether we should do it at all.

My 2¢: I'd like to try and keep the syntax of the language as simple as possible. If we can implement a new feature by introducing a new alda.lisp function, we should prefer to do it that way, vs. introducing a new literal syntax.

Great reference on ornaments: https://en.wikipedia.org/wiki/Ornament_(music)