Remove Mutable::Result and Mutable::Option #1817

Job	Run time
nightly	3m 13s
docs	2m 55s
msrv	1m 31s
clippy	2m 22s
rustfmt	13s
basics	1s
wasm	1m 11s
build_feature (alloc)	3m 23s
no_std (ubuntu-latest, minimal)	1m 9s
test_doc	6m 13s
build_feature (cli)	3m 59s
no_std (macos-latest, minimal)	1m 10s
test_lib	4m 18s
build_feature (cli,doc)	3m 52s
build_feature (cli,fmt)	3m 54s
test_miri (rune)	3m 38s
test_rune	2m 51s
test_miri (rune-core)	43s
build_feature (doc)	3m 38s
test_miri (rune-alloc)	6m 6s
build_feature (workspace)	3m 36s
build_feature (languageserver)	3m 55s
build_feature (byte-code)	3m 20s
build_feature (capture-io)	3m 26s
build_feature (emit)	3m 18s
	1h 13m 55s

Provide feedback