Partially specialize `simd` for scoped enumerations

[mattkretz]

Given simd<T, A> where T is an scoped enum (U is a shorthand for std::underlying_type_t<T>):

• [x] mask_type is simd<U, A>::mask_type
• [x] implicit broadcast ctor from T
• [x] explicit cvt ctor from simd<U, A> (enables explicit broadcast from U and int)
• [ ] P????: explicit cvt ctor from simd<T1, A1>, requires simd_size_v<T, A> == simd_size_v<T1, A1>
• [x] generator ctor, where generator return type must be T
• [x] explicit load ctor and member function from const T* and const U*
• [x] store member function to T* and U*
• [x] subscript access to elements of T
• [x] explicit conversion operator to simd<U, A>
• [ ] comparison operators ==, !=, <, <=, >, and >= returning mask_type
  • one of the operands has to be simd<T, A>, the other operand has to be convertible to simd<U, A> (which includes simd<T, A>)
  • program-defined non-member overloads of these operators take precedence
• [ ] operator?:(mask_type, const simd<T, A>&, const simd<T, A>&)
  • blends two objects of type simd<T, A> (one operand can be T, because of the implicit conversion)
  • program-defined non-member overloads of the mask_type conditional operator take precedence
• [ ] all constructors and member functions are constexpr except loads and stores
• [ ] not defined: (can be provided by program-defined overloads)
  • compound assignment
  • increment and decrement
  • unary and binary (except comparison) operators