Is flags enum needed in Phobos?

[Denis Shelomovskij]

.NET has FlagsAttribute, Java has EnumSet. Looks like we need this too. 
How about to add a library solution to Phobos?

My variant is here (search for `flagEnum`):
https://bitbucket.org/denis_sh/misc/src/tip/stdd/typecons.d

It has a bug and I have no idea how to fix it:
`AB.init |= AB.a` and `AB.a |= AB.a` are allowed.
(no, we can't make `AB.a` const to disallow the second case because it 
will disallow this: `auto a = AB.a; a |= AB.a;`)

Also I'm not sure:
* Should we support converting from a number to a flag enum?
* If so, should we support values not from enum flags or throw 
exceptions (it may be configurable)?
* Is `flagEnum` an appropriate name?



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Денис В. Шеломовский
Denis V. Shelomovskij