Proposed additional sql-ops for OVER, PARTITION-BY, WITH, WITH-RECURSIVE and WINDOW

Marijn,

As mentioned on the postmodern mailing list, this is a pull request for some proposed new sql-ops for OVER, PARTITION-BY, WITH, WITH-RECURSIVE and WINDOW with proposed amendments to existing ORDER-BY and SELECT. The change to select merely adds the WINDOW term. The change to ORDER-BY allows it to operate when applied to a single form with no additional args.

I made the modification to PARTITION-BY as you suggested to handle the ORDER-BY case. Note that in this case ORDER-BY is just a keyword, not an sql-op. You may decide that the asymmetry to be unacceptable (within a partition-by form, order-by is a keyword; outside a partition-by form, order-by is an sql-op requiring parens. This is noted in the documentation.
 
The pull request also include documentation including usage examples matching the usage examples from the postgresql website documentation.

Sabra