make `Rich` compare spans instead of all members for `Eq`

[alex-s168]

from the current code:

#[derive(Clone, PartialEq, Eq, Hash)]
pub struct Rich<'a, T, S = SimpleSpan<usize>> {
    span: S,
    reason: Box<RichReason<'a, T>>,
    context: Vec<(RichPattern<'a, T>, S)>,
}

this is annoying because T might not always implement Eq and PartialEq

for example proc_macro2::TokenTree doesn't

[alex-s168]

actually there is another problem: LabelError needs PartialEq on tokens too...

[zesterer]

Is there something specific you need PartialEq for?

[alex-s168]

Is there something specific you need PartialEq for?

No, but LabelError needs PartialEq on tokens

[zesterer]

I am happy to accept a PR for this change!