C++17 numeric interval

(ranged-based for loop friendly)

• Basic range:

    for(auto x : range(0,10) )
    { /* ... */ }  //  0,1,2,3,4,5,6,7,8,9
  

• Custom step:

   for(auto x : range(-10, 10, step{2}) )
   { /* ... */ }  //  -10,-8,-6,-4,-2,0,2,4,6,8
  

• Custom constexpr step:

   for(auto x : range(0, 20, int_step<3>{}) )
   { /* ... */ }  //  0,3,6,9,12,15,18
  

• Custom lambda step:

   for(auto x : range(1, 20, [](auto& v){ v = 2*v; }) )
   { /* ... */ }  //  1,2,4,8,16
  

• Other types:

   for(auto x : range(0.5f, 5.3f, step{0.7f}) )
   { /* ... */ }  // 0.5,1.2,1.9,2.6,3.3,4.0,4.7
  

• Custom comparison - inclusive right border:

   for(auto x : range(5, 10, step{1}, std::less_equal{}) )
   { /* ... */ }  //  5,6,7,8,9,10
  

• Custom comparison - reversed traversal:

   for(auto x : range(10, 0, step{-1}, std::greater{}) )
   { /* ... */ }  //  10,9,8,7,6,5,4,3,2,1
  

The last example can be somewhat improved syntactically (default to std::greater depending on step).