Factor out `IncomingToken` #5221

Job	Run time
test on freebsd	3m 52s
test on netbsd	4m 56s
lint	1m 3s
audit	20s
msrv	1m 5s
test (ubuntu-latest, stable)	1m 32s
test (ubuntu-latest, beta)	1m 10s
test-android (x86_64-linux-android, x86_64, 26)	2m 50s
test-aws-lc-rs	1m 56s
test (macos-latest, stable)	2m 1s
test on illumos	7m 21s
test-android (i686-linux-android, x86, 26)	2m 51s
test (windows-latest, stable)	2m 47s
test on solaris	11m 23s
test wasm32-unknown-unknown	51s
features (ubuntu-latest)	1h 24m 48s
features (macos-latest)	29m 21s
features (windows-latest)	48m 19s
	3h 28m 26s

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