New iterator method 'try_fold' can be optimized

[ExpHP]

Either can gain even more optimized iterator methods by overriding the try_fold method added in 1.27; however, this will cause a pretty big bump in the minimum supported rust version (which appears to be 1.12 based on CI test configuration).

[ExpHP]

Oops! No it cannot, because it requires mentioning the unstable Try trait! 😛

I guess I'll leave this here though for once it actually can be done. (that time must be approaching not too long from now!)

[cuviper]

We could use a build script to enable an internal cfg when Try is available.