postgres-lambda-diff

Regression tests and documentation have both been updated.

SQL2003 requires that both ceiling() and ceil() be present, so I have
documented both spellings. SQL2003 doesn't mention pow() as far as I
can see, so I decided to replace pow() with power() in the documentation:
there is little reason to encourage the continued usage of a function
that isn't compliant with the standard, given a standard-compliant
alternative.

RELEASE NOTES: should state that pow() is considered deprecated
(although I don't see the need to ever remove it.)