Convert opcode TH code to the use of Field type

This makes more explicit the field behaviour - previously an optional
field was detected via a "Maybe" constructor, and an optional one via
a "Just defval" one. With this, field behaviour become more explicit
than auto-deduced.

In THH.hs, I slightly changed the fieldVariable function to use the
field name (if the field is not renamed), so that we have the exact
same output as before.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>