PostgreSQL composite type support

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In npgsql/npgsql@a9401b1 I added composite type support, but since then EFCore changed things in a way that breaks it. It appears that the composite type is now picked up as a navigation property - this makes sense but find a way to make EFCore treat it as scalar?

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It's probably a good idea to wait and get some clarity on dotnet/efcore#246 before implementing this. Otherwise we may end up with something in Npgsql that isn't compatible with whatever the EF team decide to do with complex/value objects.

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Note: in addition to the actual mapping of the composite type, there's the question of support for creating the PostgreSQL type (i.e. CREATE TYPE).