Move more internal types to the new AnyObj abstraction

[udoprog]

There is no longer much benefit to inlining some of the built-in types in favor of moving them into the new AnyObj abstraction. Removing the Mutable enum completely will reduce complexity and in particular it will allow for more efficient type checking without the need to borrow the interior value.

See #843 which performs this change for the builtin range types.

Types that could be moved:

• String rune: Store String in AnyObj instead of Mutable (relates #844) #848
• Bytes rune: Store Bytes in AnyObj instead of Mutable (relates #844) #851
• Vec rune: Store Vec in AnyObj instead of Mutable (relates #844) #865
• Tuple rune: Store OwnedTuple in AnyObj instead of Mutable (relates #844)  #867
• Object rune: Store Object in AnyObj instead of Mutable (relates #844) #873
• ControlFlow rune: Store ControlFlow in AnyObj instead of Mutable (relates #844) #856
• Future rune: Store Future in AnyObj instead of Mutable (relates #844) #872
• Stream rune: Store Generator and Stream in AnyObj instead of Mutable (relates #844) #868
• Generator rune: Store Generator and Stream in AnyObj instead of Mutable (relates #844) #868
• GeneratorState rune: Store GeneratorState in AnyObj instead of Mutable (relates #844) #863
• Function rune: Store Function in AnyObj instead of Mutable (relates #844) #871
• Format rune: Store Format in AnyObj instead of Mutable (relates #844) #855
• Option Remove Mutable::Result and Mutable::Option #876
• Result Remove Mutable::Result and Mutable::Option #876
• EmptyStruct Simplify how variants are handled #877
• TupleStruct Simplify how variants are handled #877
• Struct Simplify how variants are handled #877
• Variant Simplify how variants are handled #877