Codegen revamp #794

Job	Run time
format	13s
test (async-std-surf)	2m 39s
clippy (async-std-surf)	1m 44s
test (tokio-hyper)	2m 50s
Docs	1m 6s
clippy (tokio-hyper)	1m 37s
verify-codegen-crate	31s
test (tokio-hyper-rustls)	2m 39s
clippy (tokio-hyper-rustls)	1m 33s
test (tokio-hyper-rustls-webpki)	2m 34s
clippy (tokio-hyper-rustls-webpki)	1m 29s
test (blocking)	2m 51s
clippy (blocking)	1m 41s
test (blocking-rustls)	2m 37s
clippy (blocking-rustls)	1m 41s
test (blocking-rustls-webpki)	2m 31s
clippy (blocking-rustls-webpki)	1m 30s
	31m 46s

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