This PR implements the new data_slice API for zero and one-sided views. Benchmarking shows a consistent 10-15% improvement in calls to to_json, but I'd love to have that validated on other machines.

Changes:

• t_data_slice allows for data extraction with bounds checking; this solves an issue with perspective-hypergrid calling get_columns with row indices that were larger than the number of rows within the engine. The use of at() instead of operator[], IIRC, should prevent undefined behavior from occurring when we index into the underlying slice. @texodus @timkpaine please look over this closely if possible.
• perspective.js now pulls column names and row paths from the data slice, instead of making calls to the underlying C++ view.
• Shared pointers are used for the data_slice and underlying slice vector, but other metadata are stored as copies and returned as const references in order to make it easier to interface with from the JS binding.