WIP: Automatic view update rules

Please find attached my current patch for automatic view update rules.
This is a stripped down version of my former patch discussed before 8.3
without CHECK OPTION support and less invasive changes to the rewriter
itself.

I'm currently cleaning up the code with all occurences of multiple base
relations (in progress) hence supporting SQL92 at the moment, only

If we decide to go further with this approach i would like to work on the
CHECK OPTION implementation based on some ideas we had in the past (e.g.
rewriter/executor support).

Note that i'm still working on this (for example, RETURNING is missing
yet), As always, discussion welcome ;)

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Thanks

Bernd