Add width_bucket(anyelement, anyarray).

This provides a convenient method of classifying input values into buckets
that are not necessarily equal-width.  It works on any sortable data type.

The choice of function name is a bit debatable, perhaps, but showing that
there's a relationship to the SQL standard's width_bucket() function seems
more attractive than the other proposals.

Petr Jelinek, reviewed by Pavel Stehule