Macro easy_flags

macro_rules! easy_flags {
    ( $(#[$onemeta:meta])* $ovis:vis $one:ident($onekw:ident) $(| $(#[$manymeta:meta])* $mvis:vis $many:ident($manykw:ident))? { $( $(#[$flagmeta:meta])* $flag:ident($flagkw:ident)),* $(,)? }) => { ... };
}

Defines flags and attribute group and fields for them. Flags can be parsed independently. They can be parsed inside attribute that can contain only one flag. They can be parsed inside attribute that accepts a list of flags if provided.