Implement `Diagnostic` for `Infallible` #924

Job	Run time
Miri	3m 57s
Minimal versions check (ubuntu-latest)	19s
Check fmt & build docs	41s
Minimal versions check (macOS-latest)	20s
Build & Test (fancy, 1.70.0, ubuntu-latest)	57s
Check Wasm build	14s
Minimal versions check (windows-latest)	38s
Build & Test (fancy, 1.70.0, macOS-latest)	47s
Build & Test (fancy, 1.70.0, windows-latest)	1m 16s
Build & Test (fancy, stable, ubuntu-latest)	44s
Build & Test (fancy, stable, macOS-latest)	47s
Build & Test (fancy, stable, windows-latest)	1m 35s
Build & Test (syntect-highlighter, stable, ubuntu-latest)	50s
Build & Test (syntect-highlighter, stable, macOS-latest)	53s
Build & Test (syntect-highlighter, stable, windows-latest)	1m 55s
	15m 53s

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