rune: Store Function in AnyObj instead of Mutable (relates #844) #1794

Job	Run time
nightly	3m 1s
msrv	1m 38s
docs	3m 4s
clippy	2m 14s
rustfmt	15s
basics	1s
test_lib	4m 8s
test_doc	6m 17s
no_std (ubuntu-latest, minimal)	1m 9s
build_feature (alloc)	3m 25s
no_std (macos-latest, minimal)	1m 12s
build_feature (cli)	3m 57s
test_miri (rune, runtime:: hir::arena:: --skip runtime::vm:: --skip runtime::vm_execution:: )	2m 27s
wasm	1m 10s
build_feature (cli,doc)	3m 41s
test_miri (rune-alloc)	5m 57s
test_rune	2m 42s
build_feature (cli,fmt)	3m 54s
build_feature (doc)	3m 40s
build_feature (workspace)	3m 24s
build_feature (languageserver)	3m 57s
build_feature (byte-code)	3m 23s
build_feature (capture-io)	3m 26s
build_feature (emit)	3m 31s
	1h 11m 33s

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