Have mutable and immutable variants

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...and remove .new for clarity.

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This should highlight the main use case, as a value object type.

I'm not really happy with using the word immutable:

• Users can add mutation in a do block and should be free to do that (so, freezeing is not an option)
• Nested values will always be mutable as long as values are not deeply frozen

Alternatives for the word immutable in this context are highly appreciated.

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Technically, .new can be replaced with two methods, one (the "immutable" semantics, see above) not adding setters. This is the only change I would introduce, and this mode should also be the default.