Add expression filter support

[tobrun]

Tailwork of expression integration in #10654. Capturing from #10654 (comment) that expression support for core needs to be implemented first.

[1ec5]

Per #7978 (comment), this task includes adding a getFilter() accessor to Layer.

[anandthakker]

Outline of an implementation approach:

1. Allow expressions to be used as filters.
   a. Add an ExpressionFilter type to include/mbgl/style/filter.hpp that holds an mbgl::style::Expression object (of type mbgl::style::expression::type::Boolean)
   b. Add a corresponding overload for mbgl::style::FilterEvaluator
   c. In src/mbgl/style/conversion/filter.cpp, add a port of isExpressionFilter and use it to decide in Converter<Filter>::operator() whether to parse the incoming JSON as an ExpressionFilter or one of the legacy filter types.
2. Convert "legacy" filters directly into expressions.
   a. Port the specialized filter-* expressions from GL JS, adding them to src/mbgl/style/expression/compound_expression.cpp.
   b. Update src/mbgl/style/conversion/filter.cpp to always convert incoming JSON values to an ExpressionFilter (using the filter-* types as needed for legacy syntax)
   c. Rewrite/simplify include/mbgl/style/filter.hpp and mbgl::style::FilterEvaluator, removing all the different *Filter types and the Filter variant and renaming ExpressionFilter to Filter.

[1ec5]

Once the core changes in #10720 (comment) are complete, we can remove the FilterEvaluator from NSExpression+MGLAdditions.mm and round out a few remaining NSPredicate-specific operators for #11013.