Make square braces only return tables, not views

[simonw]

This syntactic sugar:

db["table_name"].insert(...)

Had caused all sorts of type-related mess as it can return either a table or a view, and views can't do insert etc.

For 4.0 I can break backwards compatibility, and making it so views can no longer be returned by square brace access is a small enough breaking change that makes all of the type stuff much nicer and hence is worthwhile I think.