Limit cpo template-heads to having CPO explicit params first, then function params

[atomgalaxy]

@seanbax writes:

I think a good requirement is that all parameters deduced by the template-argument-list appear at the front of the parameter list. Then those first N params belong to the class template, and the remainder belong to the function.

G:
I see no reason to make this more convoluted than it needs to be.