Remove #[start] entrypoints (closes #897) #1875

Job	Run time
clippy	2m 14s
msrv	2m 6s
nightly	4m 41s
docs	3m 22s
rustfmt	5s
basics	1s
no_std (ubuntu-latest, minimal)	1m 21s
test_lib	4m 32s
build_feature (alloc)	3m 9s
test_doc	6m 22s
build_feature (cli)	3m 50s
build_feature (cli,doc)	3m 52s
test_miri (rune)	3m 48s
test_miri (rune-core)	1m 0s
build_feature (cli,fmt)	3m 45s
test_rune	3m 2s
no_std (macos-latest, minimal)	1m 15s
build_feature (doc)	3m 23s
test_miri (rune-alloc)	6m 22s
build_feature (workspace)	3m 3s
wasm	1m 9s
build_feature (languageserver)	3m 32s
build_feature (byte-code)	3m 4s
build_feature (capture-io)	3m 4s
build_feature (emit)	3m 1s
	1h 15m 3s

Provide feedback