Kill enable_if recipe.

This solution should work in theory, but it makes boost fall down, and
I'm not in the mood to debug why that is.
Favor manual constructor expansion over std::enable_if.

Since our too-perfect-forwarding issue only arises on single field
records (multi-field record constructors are disqualified from overload
resolution by arity), we can pay the exponential blowup for just the
single argument, which isn't bad at all.