Add FeatureWriter to stream writes

[michaelkirk]

• I agree to follow the project's code of conduct.
• I added an entry to CHANGES.md if knowledge of this change could be valuable to users.

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This allows the user to incrementally serialize a FeatureCollection, rather than requiring them to do it one-shot. This is a pretty logical counterpart to the FeatureReader introduced in #199.

[michaelkirk]

Here's an example integration into abstreet: https://github.com/a-b-street/abstreet/compare/master...michaelkirk:mkirk/geojson-writer?expand=1#diff-38d8c6286297dd3e2dac2758061359d82d7d273471512f81e839c0e2b56be395R465

This screen in ABStreet has a CSV export and a GeoJSON export - after this change, the GeoJSON generation code path looks much more like the CSV code path, and they can even share substantial parts of the code path now.

Note that one regression here, at least as far as abstreet is concerned, is that we don't currently support pretty-formatting.

/cc @dabreegster

[dabreegster]

I'm a fan of this and would it use it in a few other projects too. Thanks for improving API ergonomics so much!

Note that one regression here, at least as far as abstreet is concerned, is that we don't currently support pretty-formatting.

Not important to preserve.

[urschrei]

This is a great addition – makes perfect sense.

[michaelkirk]

bors r=urschrei

[bors]

Build succeeded:

• build_and_test