Support pub impl syntax

[taiki-e]

This replaces the impl-level visibility syntax introduced in #25 (v0.2.6). It is more natural syntax and compatible with rustfmt.

#[ext(Ext)]
pub impl Type {
    fn method(&self) {}
}

pub impl Type {
^^^

The old impl-level visibility syntax (#[ext(pub)]) will still be supported, but it is deprecated and will be removed in the next major version.

Migration:

- #[ext(pub)]
- impl Type {
+ #[ext]
+ pub impl Type {
    fn method(&self) {}
}

[taiki-e]

FYI @Boscop

[taiki-e]

bors r+

[bors]

Build succeeded:

• ci

[Boscop]

But the way to specify pub before each method will continue to work, right?

[taiki-e]

Yeah. There are currently no plans to deprecate or remove associated-item-level visibility (pub before fn/type/const).

[taiki-e]

Released in v0.2.8.