rune: Improve field operations #1851

Job	Run time
clippy	2m 10s
docs	3m 21s
msrv	1m 32s
nightly	3m 35s
rustfmt	13s
basics	1s
no_std (ubuntu-latest, minimal)	1m 5s
test_lib	4m 27s
test_miri (rune)	3m 33s
no_std (macos-latest, minimal)	1m 9s
build_feature (alloc)	3m 26s
test_miri (rune-core)	45s
build_feature (cli)	4m 9s
wasm	1m 14s
test_miri (rune-alloc)	6m 14s
build_feature (cli,doc)	4m 5s
test_doc	6m 14s
build_feature (cli,fmt)	4m 10s
build_feature (doc)	3m 44s
test_rune	2m 58s
build_feature (workspace)	3m 30s
build_feature (languageserver)	4m 0s
build_feature (byte-code)	3m 32s
build_feature (capture-io)	3m 31s
build_feature (emit)	3m 32s
	1h 16m 10s

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