Add a SFINAE friendly iterator_traits and use that instead.

[dgavedissian]

Fixes #1341 by implementing our own iterator_traits and using that instead to detect whether a type is an iterator.

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[coveralls]

Coverage remained the same at 100.0% when pulling f1080d7 on davedissian:bugfix/sfinae-iterator-traits into d2e6e1b on nlohmann:develop.

[nlohmann]

@davedissian Could you please have a look at the requested changes?

[dgavedissian]

@nlohmann Sorry, yes. Been meaning to get round to it :)

[nlohmann]

I think this is ready to merge. @theodelrieu Do you agree?

[theodelrieu]

I would prefer using, but that's a minor thing

[nlohmann]

I agree. @davedissian could you please make this last change. Then it should be good to go. Thanks!

[theodelrieu]

I added a minor comment, otherwise it looks good to me

[nlohmann]

Looks good to me.

[nlohmann]

Thanks a lot!