Add more perfect forwarding.
Allow folding move-only rvalue Records by constraining accessors.
Add variadic take and variadic split_at.
Move list/free/convert.hpp to list/convert.hpp
Add runtime and memory usage benchmarks.
Add an overload utility to the sandbox.
Make sure the non-template operators for placeholders are inlined
Low hanging optimizations in Tuple.
Provide an instance of Foldable for Products.
Add the intersperse method to List.
Undo 8076305 (constexpr-initialized lambdas).

This turned out to be pretty much useless because the call operator
still couldn't be made constexpr. Since that added a bit of complexity
without any gain, this is removed.
Change the syntax of `demux`.
Add the Enumerable type class.
Remove outdated operator_ experiment.
Instantiate Foldable and Orderable with String.
Split ext/ integral constants into different files.
Add the insert function to Set.
Add perfect forwarding in Map.