Initial draft of enhanced constructors proposal.

[stereotype441]

This proposal extends the set of actions that can be performed in the body of a non-redirecting generative constructor to include writing to non-late final fields and explicitly invoking super constructors.

This makes constructors more flexible, avoids the need for constructor initializer lists, and allows constructor augmentations to behave more consistently with function augmentations.

To preserve soundness, flow analysis is enhanced to ensure that a reference to this cannot escape from a constructor body before the object has been completely constructed.

Once this change has landed I will create the appropriate "feature" issue.

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