Added efficient iterators to magic_enum::containers::bitset

[KenanMillet]

This change introduces iterators to magic_enum::containers::bitset.
The iterators utilize std::countr_zero (or compiler-based equivalents if below c++20) to efficiently determine what the next set bit is in the bitset, allowing you to iterate over all set Enum values within a bitset much faster than:

• a magic_enum::containers::set (especially as the number of enums added to the bitset/set increases)
• and much faster than checking each bit in the bitset individually (especially as the size of the bitset increases)

The change comes with an example, but I did not see anywhere to put benchmark tests, so I left those out.