Stabilize the core::array module and reexport in std (for TryFromSliceError)

[ollie27]

TryFromSliceError has been stabilized in core::array but core::array is an unstable module. Additionally TryFromSliceError isn't re-exported anywhere in std. There isn't even a std::array module to re-export it into.

cc @rust-lang/libs

[jonas-schievink]

cc @rust-lang/release another missed libstd reexport

[Centril]

Can @rust-lang/libs confirm that you wish to stabilize TryFromSliceError in core::array?

[alexcrichton]

I suspect that we want this type stable but probably not in core::array but rather in {core,std,alloc}::slice.

[SimonSapin]

The libs team discussed this in today’s triage meeting. std::slice was suggested as an alternative location for this type, but David pointed out that array::TryFromSliceError reads better than slice::TryFromSliceError. Additionally, we expect to have more array-related APIs (e.g. iterator types) in the future. So consensus was to stabilize the array module and reexport it in std.

We feel comfortable backporting this to beta, but backporting to stable and mentioning this in a point release blog post seems excessive. This error type is part of another of a return type in another API and typically does not need to be named.

Since std::array hasn’t been formally proposed before, let’s do the process dance:

@rfcbot fcp merge