Add possibility to reference original document in single document update statements

[dothebart]

UPDATE has the possibility to directly work with a single document:

'UPDATE "dave" WITH { friends: []) } IN persons'

However, while one may use the variable emitted by a surrounding for loop:

FOR user IN users
  FILTER user._key == "7"
  UPDATE user WITH { friends: PUSH(user.friends, "8") } IN users

OLD may only be used in the RETURN statement:

'UPDATE "dave" WITH { friends: PUSH(DOCUMENT("persons/dave").friends, 8) } IN persons'

should be expressable like this:

'UPDATE "dave" WITH { friends: PUSH(OLD.friends, 8) } IN persons'

[dothebart]

https://www.arangodb.com/docs/devel/aql/data-queries.html#using-old-and-new-in-the-same-query OLD and NEW now supported.