Add ability to iterate through flags

[gsingh93]

Iterating over each each possible flag value would be a cool feature. I'm currently making an array with each constant value and then iterating over that.

[RReverser]

@alexcrichton Is this something that would be accepted in the library?

[alexcrichton]

Seems plausible!

[djg]

I was just thinking about implementing this feature for some code I'm working on, if it's not already implemented.

[Dushistov]

May be generate const array or const function that return array that contains
all values?

Then implementation of iteration will be strait forward,
plus the array may also contains names:

const fn all_values_names() -> [(Flag, &'static str); N]

This helps in implementation of custom serialization/deserialization (#147) or custom debug (#168).

What do you think?

Example of possible use-case: I want parse / generate strings like: "ab bc cd" <-> Flags::AB | Flags::BC | Flags::CD,

to generate string I need iteration plus names of constants in lower-case,
to deserialize I need names in lower-case

[KodrAus]

In #278 we merged an implementation that allows iterating over flags and their names. It effectively looks like:

pub fn iter(self) -> impl Iterator<Item = (&'static str, Self)>

It effectively does what @Dushistov described, but internally. There's still design work to be done on this though, IntoIterator to implement, and some refactoring of how we generate flags types so we can make that iter function return a concrete type instead of impl Trait so I'll keep this ticket open for now to consider those.

[KodrAus]

This is now implemented in 2.0.0-rc.1