Add FromIterator implementation for Deque

[ollien]

Fixes #683

I took the simplest possible route of just reusing the Extend implementation

[ollien]

Looks like testcfail is failing on some other branches, too, I don't believe this to be an issue with this PR.

[sgued]

Looks like testcfail is failing on some other branches, too, I don't believe this to be an issue with this PR.

Please rebase on top of main, #668 includes a fix for the MSRV CI failure.

[ollien]

@sgued Should be good to go!

[sgued]

Thank you for the PR,

This looks good and in line with the rest of the crate, but I don't really like these implementions, as they make it quite easy to have unintended panics (as you correctly test for thank you). 

We will probably remove them in the next breaking release (see #642 (comment) and #442 for discussions around that).

[ollien]

@sgued That's completely reasonable, I admit I was a bit surprised by the other panics too.

Feel free to close this out if it doesn't align with your plans.