More optimizations

Further reduced the object code size by a bit over 2 kilobytes, from
609632 bytes to 607560 on my machine (64-bit GHC 7.6.3 on Linux)

Also,  it may be worth deprecating oidValue,  as it's not terribly
relevant anymore:  the default is to create tables without OIDs,
and RETURNING is a better solution anyway.