[FEA] Use size_type in place of scalar in cudf::repeat.

[bdice]

Is your feature request related to a problem? Please describe.
Currently, cudf::tile accepts a size_type for the number of tilings, but cudf::repeat accepts a scalar const & for the number of repeats. The use of scalar seems unusual for something that has to be an unsigned integral quantity.

Describe the solution you'd like
Change API to accept size_type. Other similar APIs should be changed, too.

Additional context
@devavret @jrhemstad @davidwendt agreed with this idea.

[bdice]

I searched for instances of scalar in the code base and cudf::repeat was the only instance that I think should be changed.