Removing `proc_macro_hack`

[Ciantic]

The macros using proc_macro_hack like select!, try_join! don't work with Rust Analyzer ( rust-lang/rust-analyzer#6835 ).

I don't know what is required to remove proc_macro_hack, but given that it's page says:

Note: As of Rust 1.45 this crate is superseded by native support for #[proc_macro] in expression position. Only consider using this crate if you care about supporting compilers between 1.31 and 1.45.

It could be replaceable with proc_macro, which has better support.

[taiki-e]

we can't remove proc_macro_hack as we need to support old compilers but can use #[proc_macro] at Rust 1.45+

[taiki-e]

Released in 0.3.15.