Stop using proc-macro-hack

[cuviper]

Rust 1.45 enabled native support for #[proc_macro] in expression position, which supersedes proc-macro-hack as noted itself. However, since the decl macros in genawaiter-macro are calling proc macros in genawaiter-proc-macro, the token fix in rust-lang/rust#73293 is also needed for captured value hygiene, which was first in nightly-2020-06-25 and released in 1.46.

[cuviper]

I suppose this may be an pseudo-internal API-break between genawaiter and genawaiter-proc-macro.

[vorot93]

@whatisaphone any chance this could be merged?

[DevinR528]

@whatisaphone I'm happy to help any way I can... The change looks good to me at a quick glance.

[janpaul123]

Btw this is also causing trouble for people using rust-analyzer; see https://github.com/rust-analyzer/rust-analyzer/issues/6835#issuecomment-756648136 — so would be great to merge this :)