Remove #[start] entrypoints (closes #897) #1876

Job	Run time
clippy	2m 12s
nightly	4m 35s
msrv	1m 57s
rustfmt	6s
docs	3m 28s
basics	1s
no_std (ubuntu-latest, minimal)	1m 17s
test_miri (rune)	3m 50s
no_std (macos-latest, minimal)	1m 36s
test_miri (rune-core)	56s
test_miri (rune-alloc)	6m 15s
test_doc	6m 34s
test_lib	4m 40s
build_feature (alloc)	3m 3s
test_rune	3m 4s
wasm	1m 14s
build_feature (cli)	3m 49s
build_feature (cli,doc)	3m 44s
build_feature (cli,fmt)	3m 40s
build_feature (doc)	3m 21s
build_feature (workspace)	3m 14s
build_feature (languageserver)	3m 34s
build_feature (byte-code)	3m 8s
build_feature (capture-io)	3m 9s
build_feature (emit)	3m 5s
	1h 15m 32s

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