1) Preserve numeric numpy types as is in validator out, even if that
    numeric type is not supported as JavaScript TypedArray
 2) Update widget serializer to check numeric numpy arrays for whether
    they are compatible with TypedArrays. If not, serialize as list.
 3) Call to_pydatetime() on pandas datetime series/index values
    when passed to copy_to_readonly_numpy_array. This returns numpy
    array of datetimes (which we already know how to serialize)

Fixes datetime issue in #1160
Fixes FigureWidget issue in #1155

[skip ci]