Add geometry/range functions to support BRIN inclusion

This commit adds the following functions:
    box(point) -> box
    bound_box(box, box) -> box
    inet_same_family(inet, inet) -> bool
    inet_merge(inet, inet) -> cidr
    range_merge(anyrange, anyrange) -> anyrange

The first of these is also used to implement a new assignment cast from
point to box.

These functions are the first part of a base to implement an "inclusion"
operator class for BRIN, for multidimensional data types.

Author: Emre Hasegeli
Reviewed by: Andreas Karlsson

src/test/regress/sql/geometry.sql

@@ -79,6 +79,12 @@ SELECT '' AS twenty, b.f1 / p.f1 AS rotation
    FROM BOX_TBL b, POINT_TBL p
    WHERE (p.f1 <-> point '(0,0)') >= 1;
 
+SELECT f1::box
+	FROM POINT_TBL;
+
+SELECT bound_box(a.f1, b.f1)
+	FROM BOX_TBL a, BOX_TBL b;
+
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 -- Paths
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