point_begin/point_end should be called around xy for each Point in a MultiPoint

[michaelkirk]

Extracted from #183

When processing MultiPoints, many of the formats that implement GeometryProcessor do not call point_begin/point_end for each point in a multipoint, and instead call only xy. To be consistent with MultiLineString and MultiPolygon, we should surround each child geometry with it's corresponding begin/ends.

In truth, I don't know of a format yet that requires these events, so I expect most implementations will be a no-op, but I think making the usage consistent across Multi{Point,LineString,Polygon} makes things easier to understand.

processor.begin_multipoint(...)
for point in multipoint {
+  processor.begin_point(...)
   processor.xy(...)
+  processor.end_point(...)
}
processor.end_multipoint(...)

[kylebarron]

I don't think this is possible without a breaking change because point_begin does not have a tagged argument.