Independent seq_fwd and variant_policy

[alugowski]

Both headers are stand-alone and independent of the rest of poolSTL.

poolstl/seq_fwd.hpp

seq_fwd is a sequential policy (selectable, defaults to poolstl::seq_fwd) that simply forwards to the standard sequential (non-policy) function. Useful to write code in the parallel style even if tooling does not support it.

poolstl/variant_policy.hpp

Implements poolstl::variant_policy which supports an std::variant of policies. This used to be how poolstl::par_if was implemented (if parallel then the variant would be par, else seq. See supplement_test for an example of using this to implement a std_par_if.