Forward attributes on spec! it cases

[brianp]

Follow-up to the discussion on #1, where Bruce mentioned he'd have liked arbitrary attributes to be supported.

In attribute style (#[test_suite]) you can already put #[ignore], #[cfg(...)], #[should_panic], and so on directly on a #[test] fn, and the macro carries them through. The spec! DSL couldn't do that. There was no syntax to attach an attribute to an it, so anything that needed one meant dropping out of the DSL.

This wires it up. Attributes written before an it are parsed and emitted on the generated #[test] function:

spec! {
    mod with_attrs {
        #[should_panic(expected = "boom")]
        it "panics as expected" { panic!("boom"); }

        #[ignore]
        it "skipped unless you pass --ignored" { /* ... */ }

        #[cfg(feature = "integration")]
        it "only compiled with the integration feature" { /* ... */ }
    }
}

Works on async it too.

Attributes on hooks (before/after/before_each/after_each) are rejected with a compile error rather than silently dropped, since a hook isn't a standalone function. Gating a hook is a separate, harder thing and I left it out.

Independent of the #[after] fix in #2, so this branches straight off main.