Make single_parameter_details into a case-class/variant?

[eyalroz]

The single_parameter_details class currently has fields which are redundant/null for scalars, and other fields which are redundant/null for buffers. Let's make this struct into a case-class or variant, which can be initialized by specifying its kind (buffer/scalar) and only the nece

An alternative to doing this would be creating named constructor idioms for scalars and for buffers - which would reduce the amount of code necessary for the class itself.