Remove custom OpResult type/monad

Remove custom OpResult type/monad

Since we now have the GeneralResult as a multi-purpose monad, we can
remove the custom OpResult monad, and just use 'GeneralResult
FailMode' as our type. This allows removal of a few bits of
specialised infrastructure, relying instead on the generic one.

The restriction on using OpResult as a general monad remains as
before.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>