Expression should have public accessor for individual elements

[1ec5]

#250 made Expression.elements an internal property, leaving client code with no public way to access the individual elements in an expression other than to reimplement an Expression type to redecode the JSON data into. There should be a way to access the elements that make up an expression. The ability to introspect an expression is important for being able to transform its members, as in mapbox/mapbox-navigation-ios#2933.

/cc @mapbox/maps-ios @mapbox/navigation-ios

[1ec5]

This is not to say that must necessarily be the same public elements property as before. It would be quite sensible to expose something more semantically strict, such as separate operator and arguments properties, but there does need to be some way to generically access each of the elements in the expression.

[zugaldia]

Thanks @1ec5 - to clarify, does this ticket block you work over at mapbox/mapbox-navigation-ios#2933?

[1ec5]

Yes, it does, though that isn’t the only reason a developer might want to introspect an expression.