Improve darling::util::Flag

[TedDriggs]

Right now, Flag has some issues that limit its usefulness:

1. It has no span information, so it's impossible to create good errors on a Flag
2. It hides its optionality, which means Option handling is verbose #161 would need special logic to infer that a Flag can be implicitly defaulted.

Proposed API

Deprecate darling::util::Flag in favor of... Word?

pub struct Word(Span);

impl Spanned for Word {}

This would then be used in conjunction with Option:

#[derive(FromMeta)]
pub struct Example {
    do_things: Option<Word>
}

Note that unlike (), the implementation of FromMeta for Word would capture the span of the path:

#[example(do_things)]
//        ^^^^^^^^^

This enables useful attachment of diagnostics to the word.

Open Questions

1. Should we rename Flag in place and take the breaking change?
2. Should we have a similar utility for bool, maybe called Toggle?