NEXT VALUE FOR <sequence>

Hi

SQL:2003 introduced the function NEXT VALUE FOR <sequence>. Google
tells me that at least DB2, SQL Server and a few niche databases
understand it so far. As far as I can tell there is no standardised
equivalent of currval and setval (but I only have access to second
hand information about the standard, like articles and the manuals of
other products).

Here is a starter patch to add it. To avoid a shift/reduce conflict,
I had to reclassify the keyword NEXT. I admit that I don't fully
understand the consequences of that change! Please let me know if you
think this could fly.

Best regards,
Thomas Munro