Favor manual constructor expansion over std::enable_if.
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.

yliu120/K3,DaMSL/K3,DaMSL/K3