Replace push_str(&format!(...)) pattern in generate_full_calendar_ical,
generate_event_ical, and generate_vevent with direct write!() into a
pre-sized String buffer.
Before: ~5N+1 heap allocations for N events (temporary Strings created
by format!(), copied into the main buffer, then dropped).
After: 1 allocation (the initial String::with_capacity). All write!()
calls format directly into the destination buffer with zero intermediate
Strings.
Dionisio
7b2a8577a9