Introduce hyper_util::server::conn::auto::upgrade::downcast (#147) #470

Job	Run time
Check Style	17s
semver	2m 27s
features	1m 21s
Test with Miri	48s
Check MSRV (1.63)	3m 3s
Test stable on ubuntu-latest	31s
Test stable on windows-latest	2m 46s
Test stable on macos-latest	32s
Test beta on ubuntu-latest	34s
Test beta on windows-latest	1m 29s
Test beta on macos-latest	44s
Test nightly on ubuntu-latest	40s
Test nightly on windows-latest	1m 27s
Test nightly on macos-latest	47s
Build docs	19s
CI is green	1s
	17m 46s

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