… that returns an iterator over all the enabled flags.Uses `iter::from_fn` to create the iterator to avoid having to add a new struct to the macro which would require a parameter specifying it's name.

When a bitflags type contains composite flags like:

```rs
bitflags! {
    struct Flags: u8 {
            const A = 0b00000001;
            const BC = 0b00000110;
    }
}
```

from_bits and from_bits_truncate would not work as expected allowing flags that are not declared.

Fixes bitflags#275

Uses an array to store all the possible combinations and checks one by one all of them until the flag or the array is empty.

Requires a higher version of rust (1.51) since it needs const generics.

Cause Serialize and Deserialize derives are used from serde:: instead of serde_derive::