Avoid expensive expression parsing when drawing symbols

[mourner]

Prior to this PR, the drawSymbols routine had to reparse any icon/text-size property expressions when performing evaluateSizeForZoom routine. This led to it taking a noticeable time on the main thread.

The main goal of the PR (first 3 commits) is to simplify the sizeData structure used for it so that it only retains the interpolation factor rather than the whole expression property, making it leaner and completely avoiding the need to parse expressions. On a sample profiling of a Streers-v11 style, it shows roughly 5-10% less time for drawSymbols.

The last commit just simplifies the code to make it easier to read.

The Paint/LayerSymbol benchmarks don't show any difference because the overhead only appear when using expression-heavy styles such as Streets-v11. We should update our benchmark to reflect the current style spec usage in a separate PR.

[ansis]

Looks good! The benchmark improvement would be good as well