unfeature-gate tuple-indexing, then replace tuple indexing traits / methods accordingly

[thestinger]

These traits have been marked #[stable] so this is a backwards compatibility issue, but it will be trivial to replace these.

[alexcrichton]

Note that as-is today we can't deprecate these traits due to tuple indexing being behind a feature gate. In other words, this issue may want to rather be a simultaneous execution of un-feature-gating and deprecating the tuple traits.

[pnkfelix]

As @alexcrichton says, with tuple-indexing behind a feature gate, it would be bad to remove the tuple-accessor traits/methods until it has been unfeature-gated.

So we are planning to leave the stable markers on the tuple accessor traits unless/until we presumably unfeature-gate tuple-index at some hypothetical point in the future.

[pnkfelix]

P-high

[orium]

Does the community have consensus on making tuple_indenxing accepted (i.e. ungated)? I have seen the RFC but found no info about that.

[orium]

Ok, I just found out about this RFC 450 which ask to un-gate tuple indexing, thus blocking this issue.