rune: Improve field operations #1850

Job	Run time
clippy	2m 16s
nightly	3m 33s
docs	3m 18s
msrv	1m 35s
rustfmt	15s
basics	1s
wasm	1m 16s
test_lib	4m 36s
no_std (ubuntu-latest, minimal)	1m 5s
test_miri (rune)	3m 34s
test_rune	3m 2s
no_std (macos-latest, minimal)	1m 9s
test_miri (rune-core)	47s
test_miri (rune-alloc)	6m 7s
test_doc	6m 38s
build_feature (alloc)	3m 29s
build_feature (cli)	4m 8s
build_feature (cli,doc)	4m 8s
build_feature (cli,fmt)	4m 1s
build_feature (doc)	3m 42s
build_feature (workspace)	3m 27s
build_feature (languageserver)	4m 10s
build_feature (byte-code)	3m 40s
build_feature (capture-io)	3m 34s
build_feature (emit)	3m 31s
	1h 17m 2s

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