GeometryStreamBuilder

[kylebarron]

Change list

• Instead of using really complex code to figure out which underlying array to push each individual xy call to, we use an underlying GeoWriter to collect each geometry into a scratch buffer, and then push that geometry using our existing push_geometry APIs. This simplifies the code a lot.
• It may have a negative performance impact because we're allocating extra heap buffers for the small vecs. In the future, if we want to optimize this, we may want to create a version of geo-types that uses something like bumpalo

Closes #365