scsynth: support for receiving nested OSC bundles.

scsynth right now cannot receive nested OSC bundles, this patch adds
support for it. To play nicely with the current way of scheduling
bundles, it does so by unrolling(unnesting) nested bundles  before
handing them over via the FIFO to the realtime thread.

the example incoming packet with nested bundles:

[ #bundle IMMEDIATE
	[ /tick, 13]
	[ #bundle IMMEDIATE
		[ /s_new, 1000, ...]
	]
	[ /tock, 14]
	[ #bundle +1ms
		[ /n_set, 1000, 'gate', 1, ...]
	]
]

is unrolled to three individual packets each consisting of a single bundle:

[ #bundle IMMEDIATE
	[ /tick, 13]
	[ /tock, 14]
]

[ #bundle IMMEDIATE
	[ /s_new, 1000, ...]
]

[ #bundle +1ms
	[ /n_set, 1000, 'gate', 1, ...]
]

Signed-off-by: Hanspeter Portner <agenthp@users.sf.net>