Implement the width_bucket() function, per SQL2003. This commit only adds

Implement the width_bucket() function, per SQL2003. This commit only adds
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.