rune: Store Object in AnyObj instead of Mutable (relates #844) #1799

Job	Run time
msrv	1m 34s
rustfmt	12s
docs	3m 15s
clippy	2m 3s
nightly	3m 6s
basics	1s
wasm	1m 13s
no_std (ubuntu-latest, minimal)	1m 2s
no_std (macos-latest, minimal)	1m 6s
test_doc	6m 1s
build_feature (alloc)	3m 31s
test_lib	4m 7s
build_feature (cli)	4m 6s
test_miri (rune, runtime:: hir::arena:: --skip runtime::vm:: --skip runtime::vm_execution:: )	2m 35s
build_feature (cli,doc)	3m 57s
test_rune	2m 49s
test_miri (rune-alloc)	6m 13s
build_feature (cli,fmt)	3m 50s
build_feature (doc)	3m 34s
build_feature (workspace)	3m 15s
build_feature (languageserver)	3m 45s
build_feature (byte-code)	3m 36s
build_feature (capture-io)	3m 28s
build_feature (emit)	3m 17s
	1h 11m 36s

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