Change the RFC header to improve the PR/issue links

[chris-morgan]

(a) At present, the format of the RFC header (excluding the start date line which is fine) is like this:

- RFC PR #: 19
- Rust Issue #: 13231

• RFC PR #: 19
• Rust Issue #: 13231

(b) Given that we are using GitHub, this would be more immediately useful like this:

- RFC PR: #19
- Rust Issue: mozilla/rust#13231

• RFC PR: Opt-in builtin traits #19
• Rust Issue: Tracking Issue for auto traits (auto_traits) -- formerly called opt-in built-in traits (optin_builtin_traits) rust#13231

(c) For optimal clarity and portability across GitHub, we could extend the RFC PR to be an absolute reference:

- RFC PR: rust-lang/rfcs#19
- Rust Issue: mozilla/rust#13231

• RFC PR: Opt-in builtin traits #19
• Rust Issue: Tracking Issue for auto traits (auto_traits) -- formerly called opt-in built-in traits (optin_builtin_traits) rust#13231

(d) For absolute portability, we could use URLs:

- RFC PR: https://github.com/rust-lang/rfcs/pull/19
- Rust Issue: https://github.com/mozilla/rust/issue/13231

• RFC PR: Opt-in builtin traits #19
• Rust Issue: https://github.com/mozilla/rust/issue/13231

(e) Possibly with friendly labels (mix this with the first three options to produce (e.a), (e.b) and (e.c); below, (e.c) is shown):

- RFC PR: [rust-lang/rfcs#19](https://github.com/rust-lang/rfcs/pull/19)
- Rust Issue: [mozilla/rust#13231](https://github.com/mozilla/rust/issue/13231)

• RFC PR: rust-lang/rfcs#19
• Rust Issue: mozilla/rust#13231

I suggest (c) or (e.c).

[lilyball]

👍