rune: Store Future in AnyObj instead of Mutable (relates #844) #1797

Job	Run time
clippy	2m 11s
rustfmt	15s
docs	3m 4s
nightly	3m 22s
msrv	1m 32s
basics	1s
test_miri (rune, runtime:: hir::arena:: --skip runtime::vm:: --skip runtime::vm_execution:: )	2m 34s
test_miri (rune-alloc)	6m 3s
wasm	1m 10s
no_std (ubuntu-latest, minimal)	1m 9s
build_feature (alloc)	3m 35s
no_std (macos-latest, minimal)	1m 33s
build_feature (cli)	4m 0s
test_doc	6m 15s
build_feature (cli,doc)	3m 55s
test_lib	4m 18s
build_feature (cli,fmt)	3m 57s
test_rune	2m 52s
build_feature (doc)	3m 31s
build_feature (workspace)	3m 23s
build_feature (languageserver)	3m 54s
build_feature (byte-code)	3m 23s
build_feature (capture-io)	3m 22s
build_feature (emit)	3m 25s
	1h 12m 44s

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