rune: Store Object in AnyObj instead of Mutable (relates #844) #1800

Job	Run time
clippy	2m 12s
rustfmt	13s
docs	3m 14s
msrv	1m 37s
nightly	3m 21s
basics	1s
test_miri (rune, runtime:: hir::arena:: --skip runtime::vm:: --skip runtime::vm_execution:: )	2m 39s
no_std (ubuntu-latest, minimal)	1m 7s
test_miri (rune-alloc)	5m 56s
build_feature (alloc)	3m 24s
no_std (macos-latest, minimal)	1m 17s
test_doc	6m 7s
build_feature (cli)	3m 58s
test_lib	4m 1s
build_feature (cli,doc)	4m 0s
test_rune	2m 57s
wasm	1m 7s
build_feature (cli,fmt)	4m 4s
build_feature (doc)	3m 42s
build_feature (workspace)	3m 40s
build_feature (languageserver)	3m 55s
build_feature (byte-code)	3m 33s
build_feature (capture-io)	3m 31s
build_feature (emit)	3m 20s
	1h 12m 56s

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