This PR adds a const associated value on the VTabModule trait,
`READONLY` defaulted to `true`, so we can bail early when a write
operation is done on an invalid vtable.
This prevents extensions from having to implement `insert`,`update`,
`delete` just to return `Error::ReadOnly`, and prevents us from having
to step through `VUpdate` just to error out.
Reviewed-by: Jussi Saurio <jussi.saurio@gmail.com>
Closes#2247
92a10f94d8