Rework expression api

[tobrun]

This PR reworks the expression API to remove the generic type (for context, see #11028).

[tobrun]

@jfirebaugh @anandthakker

I made two proposal to address the comments from #11028

• https://github.com/mapbox/mapbox-gl-native/tree/tvn-rework-expression-api
• https://github.com/mapbox/mapbox-gl-native/tree/tvn-rework-expression-api-2

The former branch (this PR) removes the generic type. For api we expose that an expression can take another expression as input + we expose some facility methods for simple expressions to remove the necessity of using literal():

eg.

get(literal("name")) is shortened to get("name")

The latter branch reworks the API to remove all types and relies on using object.java. This makes the api a little less discoverable but has the benefit of not needing to wrap literals in complex expressions.

eg.

match expression from this PR

        match(
          get("name"),
          literal("Westerpark"), rgb(255, 0, 0), 
          literal("Jordaan"), rgb(0, 255, 0),
          literal("Prinseneiland"), rgb(0, 0, 255),
          rgb(255, 255, 255)
        )

match expression from second proposal

        match(
          get("name"),
          "Westerpark", rgb(255, 0, 0),
          "Jordaan", rgb(0, 255, 0),
          "Prinseneiland", rgb(0, 0, 255),
          rgb(255, 255, 255)
        )

Thoughts/Preferences?

cc @ivovandongen @LukasPaczos