docs: remove mention to nightly

[hackaugusto]

Return position impl was enabled in Rust 1.26 in May 10, 2018 [1].

1- https://blog.rust-lang.org/2018/05/10/Rust-1.26.html#impl-trait

[Marwes]

Oh, man that has been there for a while. I guess I forgot since the macro really isn't that useful anymore (outside of combine itself I suppose, since it is nice to name the parser types in case someone needs them).

[hackaugusto]

CI is failing for something unrelated. Let me know if I should rebase this.

 error[E0635]: unknown feature `proc_macro_span_shrink`
  --> /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.52/src/lib.rs:92:30
   |
92 |     feature(proc_macro_span, proc_macro_span_shrink)
   |                              ^^^^^^^^^^^^^^^^^^^^^^

[Marwes]

If you do a cargo update and push the updated lockfile I think it should fix itself (seen it before).

[hackaugusto]

If you do a cargo update and push the updated lockfile I think it should fix itself (seen it before).

done :)

[Marwes]

Seems like memchr requires more recent rust versions trying to fix it in #360

[hackaugusto]

Seems like memchr requires more recent rust versions trying to fix it in #360

rebased this PR over the new master