what template arguments should `simd_cast`s take?

[mattkretz]

// Option 1:
auto a = datapar_cast<fixed_size_datapar<double, floatv::size()>>(x);
// Option 2:
auto b = datapar_cast<double>(x);
// Option 3:
auto c = datapar_cast<fixed_size_datapar<double, floatv::size()>>(x);
auto d = datapar_cast<double>(x);