allow fields to represent top level attributes

[ModProg]

i.e.:

#[derive(FromAttr)]
#[attribute(flatten)]
struct FieldAttribute {
    default: FlagOrValue<Expr>,
    children: bool,
}

would parse

#[default]
#[default = <expr>]
// This is supported additionally, because
// `=` only supports a subset of expressions
#[default(<expr>)]

#[children]

We could support this maybe as a fieldlevel attribute as well, or allow the field value to be anything that implements FromAttr to allow hybrid structs with nested and unnested attributes.