a variant of the function for the 'numeric' datatype; it would be possible
to add additional variants for other datatypes, but I haven't done so yet.
This commit includes regression tests and minimal documentation; if we
want developers to actually use this function in applications, we'll
probably need to document what it does more fully.
float8 types. This begins the deprecation of this feature: in 7.6,
this input will be rejected.
Also added a new error code for warnings about deprecated features,
and updated the regression tests.