Make BoxPredicate::find_case use the inner find_case #244

Job	Run time
Docs	19s
Minimal versions (ubuntu-latest)	20s
rustfmt	15s
Check MSRV (ubuntu-latest)	21s
Test (ubuntu-latest, stable)	33s
clippy	32s
lockfile	9s
Test (macos-latest, stable)	38s
Coverage	23s
Test (windows-latest, stable)	1m 14s
CI	2s
	4m 46s

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