Coerce selected Arrow types in vgplot.

[jheer]

• Map Arrow Timestamp types to JS Date objects, rather than numbers.
• Map Arrow Int64 types to JS Number instances, rather than BigInt. (FWIW, Plot already does this internally.)

These changes apply to unpacking of Arrow tables within vgplot only. We unpack an Arrow table into an array-of-objects format. This was already happening implicitly (using possibly slower Proxy objects); we now do it explicitly with control over type conversions. In the future, if Plot adds support for columnar data we could modify our approach to take advantage of that and reduce data copy costs.

Mosaic client implementations still receive a standard, unaltered Arrow table. This PR targets vgplot mark internals.

Possible future work:

• coerce other types, such as Time values?
• refactor coercion support to a utility library for other client implementations to use if desired.

Close #155.
Close #176.