Optimize and simplify symbol placement code

[mourner]

While I'm out of power and Internet, I'm going to be working offline on optimizing and simplifying symbol placement code, which is terrifyingly complex at the moment and in dire need of cleanup. Will try to do that in self-contained commits and push periodically when I'm back online.

Results

Zooming around z14–16 on streets-v11 SF with lots of roads in view, looking at profiler results, this PR seems to improve performance of placement along lines by ~40–50%.

Before:

After:

Benchmarks don't show a convincing win, just tiny improvements close to statistical noise — I guess they don't hit label-dense areas enough, but it's still an improvement (most diagnostic metrics show a positive change, even if tiny):

• rendering-2d
• rendering-3d
• loading
• style-weighted-all

The PR also cuts down the bundle size a little (-314 bytes) and improves flow types in a few places around symbol code (replacing any with proper typing).

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality
• post benchmark scores
• manually test the debug page
• apply changelog label ('bug', 'feature', 'docs', etc) or use the label 'skip changelog'
• add an entry inside this element for inclusion in the mapbox-gl-js changelog: <changelog>Improve performance of text and icon placement.</changelog>

[mourner]

Hold on while I'm fixing up the last commit...

[karimnaaji]

A first pass! I still have to carefully go through more of the changes. But this looks really good to me already, a lot of nice simplifications.

[karimnaaji]

I finished going over the change and left a few more minor comments/questions. But this was a really needed cleanup and glad to see that we can get out some noticeable performance gain at the same time, LGTM!