Make `self` receiver and `start_fn` members available as official fields #843

Job	Run time
cargo-fmt	20s
cargo-lock	6s
cargo-machete	7s
cargo-doc (ubuntu)	24s
cargo-miri	1m 38s
cargo-doc (windows)	1m 18s
compilation-benchmarks	1m 57s
cargo-doc (macos)	40s
prettier	8s
runtime-benchmarks (args_3)	56s
runtime-benchmarks (args_5)	1m 10s
taplo-fmt	3s
test-stable (ubuntu)	1m 19s
runtime-benchmarks (args_10_alloc)	1m 0s
test-msrv (ubuntu)	1m 27s
test-stable (windows)	4m 37s
runtime-benchmarks (args_10_structs)	54s
test-msrv (windows)	3m 1s
test-stable (macos)	1m 41s
test-unstable (beta)	31s
runtime-benchmarks (args_10)	53s
test-msrv (macos)	1m 21s
test-unstable (nightly)	31s
runtime-benchmarks (args_20)	50s
typos	6s
version-sync	4s
website-build	23s
	27m 25s

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