Add conversions between enumerations and associated values

[1ec5]

Added protocol-based initializers to Geometry and GeoJSONObject that accept each case’s associated value as an initializer argument. This makes the enumerations more consistent with the JSONValue and FeatureIdentifier enumerations and simplifies some common workflows.

JSONValue and FeatureIdentifier now conform to the Hashable protocol. These types are likely to be used as dictionary keys or set members, and it’s pretty easy to make them hashable.

Fixes #158.

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

[macdrevx]

I feel really comfortable with the GeometryConvertible and GeoJSONObjectConvertible portions of this PR. It's less obvious whether the JSONValueConvertible portions are what we want. If there's a way to defer those parts, I think it'd be worth taking the extra time.

[1ec5]

I split out the JSONValueConvertible changes as #164, since there are still open questions about the extra syntactic sugar it would introduce.