Added indexable properties on enums

[Shelim]

• _to_index which always return value 0... size-1
  (even if enums are not sequential)
• _from_index used for other way around
• _from_index_nothrow no throw version
• _from_index_unchecked returns invalid enum
  in case arg>=size

Code and test cases added.
No documentation updates (my English is too poor for that)

Usage case:

BETTER_ENUM(Enum, int, A = 128, B = 256, C = A)
for(int i = 0; i < Enum::_size(); i++)
{
    Enum foo = Enum::_from_index_unchecked(i);
    printf("%d enum is %s \n", i, foo._to_string());
}