Mono Trigger Mode

[emuell]

Adds a new "mono" trigger mode to rhythm.

"external_data" in context now is an explicit "trigger_event" which is passed down to Lua callbacks as polyphone note event.

[github-actions]

Benchmark for 855cca5

Click to view benchmark

Test	Base	PR	%
Cycle/Generate	48.6±1.15µs	49.3±0.47µs	+1.44%
Cycle/Parse	355.8±9.59µs	360.1±3.97µs	+1.21%
Rust Phrase/Clone	438.1±6.15ns	439.4±5.64ns	+0.30%
Rust Phrase/Create	71.2±1.08µs	72.2±1.21µs	+1.40%
Rust Phrase/Run	640.5±13.12µs	651.0±42.25µs	+1.64%
Rust Phrase/Seek	146.3±271.70µs	155.4±276.07µs	+6.22%
Scripted Phrase/Clone	615.4±6.47ns	616.9±9.55ns	+0.24%
Scripted Phrase/Create	963.9±31.87µs	965.1±21.00µs	+0.12%
Scripted Phrase/Run	1679.4±16.85µs	1662.8±42.10µs	-0.99%
Scripted Phrase/Seek	219.9±445.07µs	212.8±425.84µs	-3.23%