Add Scan to pgtype.FlatArray

[regeda]

Make pgtype.FlatArray respectful to sql.Scanner interface which makes sense when the standard sql package in use.

See #1779

[jackc]

This seems to have broken a couple tests. Not immediately obvious why.

But even without that I'm not sure if this is the right approach. I would expect it to work with most of the built-in types, but not all. And if the user has customized their pgtype.Map it would mysteriously fail.

There's a fundamental issue that the with database/sql we don't have access to the type information from the database. And with a generic array type we don't have access to a specific Go type that we know corresponds to a specific PostgreSQL type.

I suspect that the solution will have to be in the stdlib package either as concrete types for each array type or a generic array type that requires the underlying type to have an additional method.