Implement `IntoIterator` for fixed size arrays

[nrc]

Size 0 to 12 or whatever.

Because otherwise it is only implemented via autoref and then you get references to the contents where you want the contents by value.

[Gankra]

psuedo-blocked on ManuallyDrop rust-lang/rfcs#197

There's no way to soundly partially move out of an array in a zero-cost way today. @bluss' ArrayVec crate accomplishes this by putting the array in a modified Option (one that supresses the null ptr optimization) and ptr::writeing None in the dtor.

[Gankra]

We can certainly do the ArrayVec thing to get the job done if it's dire.

[bluss]

This would be a breaking change since [1, 2, 3].into_iter() compiles in Rust 1.4 (and is the same as calling .iter()). However it's a minor change by being avoidable with more specific/UFCS method calls.

Fortunately for _ in [1, 2, 3] does not compile in Rust 1.4.

[Gankra]

Seems like acceptable breakage, since this is clearly desirable.

[steveklabnik]

This is a duplicate of #25725