Refactor the `Warning` type #611

Job	Run time
signet	1m 14s
bitcoind	2m 30s
build (ubuntu-latest, 1.63.0)	2m 57s
build (ubuntu-latest, stable)	2m 30s
build (ubuntu-latest, beta)	2m 52s
build (ubuntu-latest, nightly)	2m 32s
build (macos-latest, 1.63.0)	2m 38s
build (macos-latest, stable)	2m 4s
build (macos-latest, beta)	2m 31s
build (macos-latest, nightly)	1m 51s
build (windows-latest, 1.63.0)	3m 39s
build (windows-latest, stable)	4m 23s
build (windows-latest, beta)	3m 32s
build (windows-latest, nightly)	3m 33s
	38m 46s

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