rune: Store Function in AnyObj instead of Mutable (relates #844) #1795

Job	Run time
msrv	1m 30s
docs	3m 15s
nightly	3m 14s
clippy	2m 2s
rustfmt	12s
basics	1s
wasm	1m 11s
test_miri (rune, runtime:: hir::arena:: --skip runtime::vm:: --skip runtime::vm_execution:: )	2m 39s
test_miri (rune-alloc)	6m 6s
no_std (ubuntu-latest, minimal)	1m 10s
build_feature (alloc)	3m 23s
no_std (macos-latest, minimal)	1m 6s
test_doc	6m 18s
test_lib	4m 9s
build_feature (cli)	4m 4s
test_rune	2m 44s
build_feature (cli,doc)	4m 2s
build_feature (cli,fmt)	4m 4s
build_feature (doc)	3m 34s
build_feature (workspace)	3m 33s
build_feature (languageserver)	3m 48s
build_feature (byte-code)	3m 35s
build_feature (capture-io)	3m 29s
build_feature (emit)	3m 31s
	1h 12m 40s

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