Factor out `IncomingToken` #5216

Job	Run time
msrv	1m 6s
audit	22s
lint	1m 6s
features (ubuntu-latest)	1h 25m 34s
features (macos-latest)	28m 40s
test (ubuntu-latest, stable)	1m 30s
test-android (x86_64-linux-android, x86_64, 26)	3m 10s
features (windows-latest)	49m 31s
test (ubuntu-latest, beta)	1m 21s
test-aws-lc-rs	2m 7s
test-android (i686-linux-android, x86, 26)	2m 51s
test (macos-latest, stable)	1m 32s
test (windows-latest, stable)	5m 11s
test on freebsd	4m 21s
test on netbsd	5m 0s
test on illumos	6m 40s
test on solaris	15m 22s
test wasm32-unknown-unknown	49s
	3h 36m 13s

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