Fix cycle poly channel flattening

[unlessgames]

There was a bug in the event flattening logic that resulted in incorrectly overlapping events in the output stream for some nested polyphonic patterns which in turn supressed some events from playing downstream.

Also cleaning up empty vecs from the output.

[github-actions]

Benchmark for 62961ac

Click to view benchmark

Test	Base	PR	%
Cycle/Generate	48.6±0.76µs	48.4±0.63µs	-0.41%
Cycle/Parse	377.5±5.78µs	380.4±4.95µs	+0.77%
Rust Phrase/Clone	435.4±6.13ns	427.9±6.43ns	-1.72%
Rust Phrase/Create	75.2±1.37µs	76.7±1.16µs	+1.99%
Rust Phrase/Run	663.3±7.32µs	647.3±5.13µs	-2.41%
Rust Phrase/Seek	135.7±250.23µs	138.3±255.98µs	+1.92%
Scripted Phrase/Clone	653.3±5.41ns	651.3±5.41ns	-0.31%
Scripted Phrase/Create	989.9±16.61µs	983.8±11.27µs	-0.62%
Scripted Phrase/Run	1723.5±15.30µs	1716.4±12.17µs	-0.41%
Scripted Phrase/Seek	224.4±452.50µs	220.8±446.33µs	-1.60%

[emuell]

Looks good to me and works fine from what I've tested.