Rewrite using proc-macro-hack

[roblabla]

Hey,

It's now possible to create functionlike!() proc macros on stable rust using proc-macro-hack.

I'm going to try rewriting power-assert-rs using it.

[roblabla]

Actually, this is impossible because of dtolnay/proc-macro-hack#15.

Proc Macros are going to be stabilized on 1.30, but they won't be able to expand to expressions. Furthermore, this proc macro would rely on spans, which are also unstable still. I'll rewrite it using those feature so they work on the latest nightly.

[roblabla]

https://github.com/roblabla/power-assert-rs

here's a WIP port to Proc Macros 1.30. It has several problems:

• It hits Feature request: get access to the exact text source (&str) behind a Span dtolnay/proc-macro2#110
• It uses a doc hidden'd function from proc_macro2: Span::unstable(), in order to avoid the semver_exempt cfg flag.
• It requires two nightly features: proc_macro_hygiene and proc_macro_span

[dtolnay]

I released proc-macro-hack 0.5.0 which preserves spans and supports expanding to expressions.

[roblabla]

There’s a PR open to recover the raw string of a Span: rust-lang/rust#55780. It will be feature gated behind proc_macro_span.

Once it lands, I’ll update my fork, and it’ll likely be usable (albeit nightly only)