vtile encoding benchmark script

[springmeyer]

We have a vtile decoding benchmark (https://github.com/mapbox/mapnik-vector-tile/blob/master/bench/vtile-decode.cpp) but not an encoding benchmark.

We should:

• [ ] write one, check it into v2_spec branch
• [ ] port to master branch
• [ ] run them both against the same data
• [ ] profile and see a) which is faster, b) whether noticable overhead exists in v2_spec branch from returning tile_layer by value like here

[springmeyer]

done in https://github.com/mapbox/mapnik-vector-tile/blob/master/bench/vtile-encode.cpp