add generic nextval construct

[Mike Bayer]

i.e. next_value(some_seq), more usefully apply it to Sequence, some_seq.next_value()

this is somewhat major since we'd be using this construct internally as well for consistency.

[Mike Bayer]

patch attached includes #2084 and #2085 .

Tests needed:

• Postgresql executes a Sequence, returns nextval, whether or not "optional" is set
• Postgresql renders the "nextval" construct, whether or not "optional" is set
• func.next_value().execute()/.scalar() works
• Using a DefaultGenerator in the columns, where clause of a select, or in the values clause of insert, update, raises the informative error
• Same for Sequence, test the additional string
• can use next_value() in:
• select column expr
• whereclause
• values of insert/update
• return of next_value is an int, check oracle
• insert statement:
• inserted_primary_key contains the result when pk_col=next_value(), when implicit returning is used, when not used, it should be none. probably want to put this in test_returning
• next_value() used on non-sequence platform either raises NotImplemented, or perhaps we determine some other kind of placeholder

[Mike Bayer]

90335a89a98df23db7a3ae1233eb4fbb5743d2e8

[Mike Bayer]

Removing milestone: 0.7.0 (automated comment)