Row deltas return arrow-serialized data

[sc1f]

When calling view.on_update with the modes parameter set to row, the callback receives an arrow-serialized version of the data from the rows that were updated.

• on_update mode parameters have been changed to cell and row
• new get_data methods have been added to each context allowing for data from certain row indices to be returned.
• to_arrow and col_to_js_typed_array now take options.data_slice, which specifies a custom t_data_slice object from which they will serialize.
• fill_col_bool now respects undefined and null values for arrow-serialized booleans.
• t_data_slice has a new constructor which specifies a vector of row indices
• process_subscribe in host.js now transfers arrow data and respects the arguments that are in its caller. This enables full functionality for on_update mode flags as well as the arrow returned by get_row_delta.

[texodus]

Thanks for the PR!

Very cool new feature, and Initial benchmarks look awesome! I have some fixes for issues around practically setting up this example, and an example project for this, which I'll submit in a separate PR.