rune: Store GeneratorState in AnyObj instead of Mutable (relates #844) #1752

Job	Run time
clippy	1m 45s
msrv	1m 39s
docs	3m 2s
nightly	3m 1s
rustfmt	16s
basics	1s
wasm	1m 6s
test_miri (rune, runtime:: hir::arena:: --skip runtime::vm:: --skip runtime::vm_execution:: )	2m 36s
no_std (ubuntu-latest, minimal)	1m 8s
test_miri (rune-alloc)	6m 10s
build_feature (alloc)	3m 20s
no_std (macos-latest, minimal)	1m 5s
test_doc	5m 48s
build_feature (cli)	3m 58s
test_rune	2m 19s
build_feature (cli,doc)	4m 24s
test_lib	3m 35s
build_feature (cli,fmt)	3m 57s
build_feature (doc)	3m 38s
build_feature (workspace)	4m 4s
build_feature (languageserver)	3m 48s
build_feature (byte-code)	3m 22s
build_feature (capture-io)	3m 22s
build_feature (emit)	3m 23s
	1h 10m 47s

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