[FEA] Design and implement cudf::scalar

[harrism]

Is your feature request related to a problem? Please describe.

With the first phase of the libcudf core data structure redesign in place (#2207), we need to add a scalar type, cudf::scalar to replace the legacy gdf_scalar struct.

Describe the solution you'd like

cudf::scalar needs to support all the types gdf_scalar supports, as well as string and category scalar values. I think a good test of gdf_scalar is whether it can be used to cudf::fill() a column of any type.

I'm sure @jrhemstad has thoughts on design to share.

[jrhemstad]

Unlike with column_views, for cudf::scalar, I'd like to use a derived class pattern to implement cudf::scalar. We will pass cudf::scalar into libcudf APIs via a base class const&, and then cast to the appropriate derived class via the type dispatcher.

[jrhemstad]

I'd like to effectively emulate Arrow Scalar for this.