bluss#311

Desired feature is that clone of ArrayVec<u8, 8> should look like a
simple struct copy (regardless of how full the arrayvec is).

Simplify the clone implementation while preserving this.

Scope guard for length not found to be necessary. Write element and
length possible but it is equivalent to push_unchecked which also worked
here.

Use std::hint::black_box (From Rust 1.6x), and use it more; this should
make the benchmarks at least a little bit more representative.

Keeping an eye on the benchmarks, simplify the optimization in
extend_from_iter; use a simpler scope guard. Use a counted index.
Previously there was a ZST special case which is not needed because:

`ptr` is a valid pointer for writing and `ptr.add(index)` still too;
`.write(elt)` consumes the element.

(Previous implementation: ptr was an out of bounds pointer for ZST.)

Benchmarks found to be sensitive to the order of predicates in
`if index == end && CHECK` which shouldn't be the case, but it was.

extend_from_iter: use only I: Iterator - reduce generics already in
`extend`.