[C#] NFC: split out IStructuralWrite interface

[RReverser]

Structural read implementation can be only directly implemented on types with default constructors, but writing should not have such limitations.

In this PR I'm splitting out writing into its own interface, so that it can be implemented even on types like tagged enums, which simplifies their own BSATN.Write implementation and allows to use them with helpers like ToBytes.

I'm also making the generated tagged enum record abstract - which it, arguably, should've been from the beginning - but it should be a non-functional change because its constructor has always been private anyway.

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