Add `CombineAs<R>()` parameters generator

[Stoorx]

CombineTo() allows the user to combine two or more sequences to produce values of a Cartesian product of those sequences' elements converted to the required type. The behavior of this function is the same as already implemented and well-known Combine().

CombineTo() allows to use struct-parametrized tests directly instead of dealing with tuples. Maybe the better naming for this function is CombineAs()

The behavior of other functions have not been changed (according to the tests passing). The additional basic test cases are provided for this feature. Feel free to give me some ideas to make better coverage.

Dear maintainers, please note, this feature is very necessary in GROMACS project and we want it as soon as possible. I see that 1.16.0 has been released. Maybe it is possible to make the new release faster and include this feature in the upcoming release somehow? Or maybe backport it as 1.16.1 too?