Let library users handle output however they like, and add flatgeobuf output

[dabreegster]

3 functional changes here:

1. Change the public API to output features through a callback, letting the caller do whatever they like. Closes Revisit library API and consider streaming #50.
2. Add an option to output the result as flatgeobuf instead of geojson
3. Huge speedups -- from 58s to 22s for one large benchmark -- just from upgrading to latest geo packages!

I used the input data from #5: time cargo run --release -- disaggregate --od-csv-path ~/Downloads/od_sp_center.csv --zones-path ~/Downloads/zones_sp_center.geojson --output-path result.fgb --output-fgb

Before this PR, output to geojson took 58s. After this PR, down to 22s!

FGB output takes much longer, 50s. Not sure why yet, though I have a few ideas. The resulting file is much smaller -- 735MB FGB, 1.4GB GeoJSON. I haven't checked downstream yet to see which is faster to read, or if reading is even the bottleneck in a larger pipeline.

This is a step towards #49 (comment) improving the API for language bindings, but the input is still a bunch of files.

[dabreegster]

I considered using generators to wind up with an iterator-like API, but

1. Generators aren't in stable Rust yet, and the 3 crates I looked at were a bit unmaintained
2. Generators wouldn't make it easy for us to plumb back errors or halt execution
3. The callback is not hard at all to use