WIP: parse points without heap allocation

[michaelkirk]

This is an addendum to #222, I'm not sure it's going to work out, but I wanted to share progress in case someone smarter than me wants to weigh in...

Context: In main today, we represent coordinates (Positions) as a Vec of floats. This entails a heap allocation per point, which is slow. #222 changes Positions to be backed by a tiny_vec, which can be stack allocated up to a certain size.

However, even with #222, we still use serde_json to parse the input - so those Vec's are created before converting them to the tiny_vec backed representation.

This branch represents my various experiments to avoid allocating Vec's to parse points. I will be the first to admit that it's currently very gnarly. I am not very familiar with the innards of serde (I'm getting a little better through this work!).

Unlike #222, this branch is very far from being mergable. But as a POC I did want to share some modest wins:

Note that this bench is baselined against the changes in #222, not main:

$ cargo bench --bench="*" -- --baseline="tiny-vec"
 
parse (countries.geojson)
                        time:   [1.8220 ms 1.8453 ms 1.8728 ms]
                        change: [-2.6785% -1.9295% -1.1463%] (p = 0.00 < 0.05)
                        Performance has improved.

FeatureReader::features (countries.geojson)
                        time:   [5.2148 ms 5.2285 ms 5.2429 ms]
                        change: [-10.682% -10.259% -9.8336%] (p = 0.00 < 0.05)
                        Performance has improved.

# 👀 this input is small, so maybe not very representative, but still -
# I'm curious why the wins are so huge here while so relatively modest elsewhere.
# I'd guess that in the other benches, much of the remaining time is spent parsing floats.
parse (geometry_collection.geojson)
                        time:   [489.36 ns 490.73 ns 492.07 ns]
                        change: [-96.840% -96.814% -96.794%] (p = 0.00 < 0.05)
                        Performance has improved.

     Running benches/serialize.rs (target/release/deps/serialize-52a1ff0867a899e5)
serialize geojson::FeatureCollection struct (countries.geojson)
                        time:   [2.8810 ms 2.8919 ms 2.9034 ms]
                        change: [+1.2959% +1.7045% +2.1246%] (p = 0.00 < 0.05)
                        Performance has regressed.

     Running benches/to_geo_types.rs (target/release/deps/to_geo_types-9c7e7ca989cc32d0)
quick_collection        time:   [66.437 µs 66.623 µs 66.819 µs]
                        change: [-0.1052% +0.4353% +0.9697%] (p = 0.11 > 0.05)
                        No change in performance detected.

🚨 This WIP currently breaks lots of things - like custom struct serialize/deserialization. If I proceed with this, I'll fix that before marking it "ready for review".