Fix template instantiations and visibility

[kkaefer]

The *_impl.hpp files has defects when instantiating the promised symbols. Instead of this template specialization declaration:

template <> geometry parse<geometry>(const std::string &);

it should’ve been a template instantiation:

template geometry parse<geometry>(const std::string &);

Additionally, it introduces a few macros that allow controlling symbol visibility:

Adds MAPBOX_GEOJSON_VISIBILITY, MAPBOX_GEOJSON_RAPIDJSON_VISIBILITY, and MAPBOX_GEOJSON_VALUE_VISIBILITY. If defined (e.g. to __attribute__((visibility ("default")))) they will allow you to set the visiblity of the symbols for the *_impl.hpp files. MAPBOX_GEOJSON_VISIBILITY propagates to the other two, so if you just want parse/stringify, but e.g. not RapidJSON explicitly define the other two to be empty.

I used nm to check that the shared library includes the correct public symbols:

geojson parse(std::const string&)
geojson parse<geojson>(std::const string&)
feature_collection parse<feature_collection>(std::const string&)
feature parse<feature>(std::const string&)
geometry parse<geometry>(std::const string&)
std::string stringify(const geojson&)
std::string stringify<geojson>(const geojson&)
std::string stringify<feature_collection>(const feature_collection&)
std::string stringify<feature>(const feature&)
std::string stringify<geometry>(const geometry&)
geojson convert(const rapidjson_value&)
geojson convert<geojson>(const rapidjson_value&)
feature_collection convert<feature_collection>(const rapidjson_value&)
feature convert<feature>(const rapidjson_value&)
geometry convert<geometry>(const rapidjson_value&)
rapidjson_value convert(const geojson&, rapidjson_allocator&)
rapidjson_value convert<geojson>(const geojson&, rapidjson_allocator&)
rapidjson_value convert<feature_collection>(const feature_collection&, rapidjson_allocator&)
rapidjson_value convert<feature>(const feature&, rapidjson_allocator&)
rapidjson_value convert<geometry>(const geometry&, rapidjson_allocator&)
geojson convert(const value&)
geojson convert<geojson>(const value&)
feature_collection convert<feature_collection>(const value&)
feature convert<feature>(const value&)
geometry convert<geometry>(const value&)
value convert(const geojson&)
value convert<geojson>(const geojson&)
value convert<feature_collection>(const feature_collection&)
value convert<feature>(const feature&)
value convert<geometry>(const geometry&)