Add ceiling() as an alias for ceil(), and power() as an alias for pow().

Add ceiling() as an alias for ceil(), and power() as an alias for pow().
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.)