Add t_data_slice for data output

[sc1f]

This PR begins to abstract the process of getting data out of perspective for use in visualizations/serialization. Right now, we create an intermediate array inside emscripten.cpp that contains the data extracted from the context. The end goal of the PR is to eliminate that intermediate structure—and the logic surrounding it—so as to make the data output API portable and trivial to implement in binding languages.

t_data_slice is a class that contains the underlying slice of data and associated metadata, which allows us to eventually remove the intermediate logic of get_data in emscripten.cpp.

Changes

• Adds t_data_slice class
• Adds is_column_only to context construction
• Moves get_column_extent to independent class

Making the PR so we can discuss and get these changes into mainline before the branch gets too big; I've tried to benchmark these changes but the script seems to be broken on my machine and isn't benching properly.

[texodus]

Thanks for the PR!

This looks great! I added one small fix for the windows build due to missing std::min on Windows. Benchmarks show a ~10% improvement in JSON generation, which is quite a nice bonus!