Add `Tool::Custom` which runs an `Operation`. #206

Job	Run time
miri	1m 29s
no_std	41s
build (ubuntu, stable, locked, true)	52m 52s
build (windows, stable, locked, false)	24m 51s
build (macos, stable, locked, false)	19m 24s
build (ubuntu, nightly, locked, false)	14m 5s
build (ubuntu, beta, locked, false)	18m 44s
build (ubuntu, stable, latest, false)	19m 34s
build (ubuntu, stable, minimal, false)	3m 26s
deploy	5s
fuzz	17m 42s
	2h 52m 53s

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