Currently, the job ID is a simple type alias. This is suboptimal, as
it means we can't use a custom JSON (or Arbitrary) instance for it.

The patch changes it into a newtype, and then a) simplifies some
deserialisation code and b) changes some more fields to this new type
(rather than plain 'Int').

We also move the JobId to types, since it will be needed in opcodes as
well.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>