* [core] Add format override expression and formatted section to evaluation context
* [core] Add textColor to TaggedString's formatted section
* [core] Add FormatSectionOverrides and introduce overridable properties
* [core] Populate symbol layer paint properties for text sections
* [core] Add benchmark for style that uses text-color override
* [core] Add unit test for FormatOverrideExpression
* [core] Add unit test for FormatSectionOverrides
`LayerManager` is now responsible for `RenderLayer` instances creation,
so that there is a single entry point for creating of objects, which
correspond to a certain layer type.
The `LayerType type` field is dropped from `Layer::Impl`.
