perf gap with flexpolyline

[urschrei]

The https://docs.rs/flexpolyline/0.1.0/flexpolyline/ crate runs the bench_encode benchmark in around 19 µs 330 ns on my machine, vs 77 µs for our decode function. That's in the 3x ballpark. They seem to use a lot of lookups, which I imagine accounts for the difference, but it would be good to investigate.

[mattiZed]

I have looked into their code, I assume you're right, the speed up comes from the explicit conversions via LUTs. In their algorithm they also need it because of the more complex char-map which is not just monotonically increasing from ASCII 63 as in GPolyline. I will try the approach while fixing #39.

[urschrei]

Addressed by #42, #43, #48, #49.