Allow manipulation of etl::array in constexpr function

[Vitmark97]

This is mainly usefull for creating constexpr arrays in C++14 and up.

Example:

template <size_t SIZE>
constexpr etl::array<int, SIZE> x_squared(int start_x) {
  etl::array<int, SIZE> out{};
  etl::iota(out.begin(), out.end(), start_x);
  for (int& elem : out) {
    elem = elem * elem;
  }
  return out;
}

// Contains [25,16,9,4,1,0,1,4,9,16,25]
constexpr etl::array compile_time_array = x_squared<11>(-5);

// Contains [35,26,19,14,11,10,11,14,19,26,35]
constexpr etl::array lambda_generated_array = []() constexpr {
  etl::array out{compile_time_array};
  for (size_t i = 0; i < out.size(); ++i) {
    out[i] = out[i] + 10;
  }
  return out;
}();