Optimized implementation for `from_header_and_slice`

[vkrasnov]

In my code I use ThinArc<(), u8> a lot, and apparently the compiler can't optimize the iterator in from_header_and_iter away. It really does iterate over each byte and writes it using ptr::write. I implemented an optimization that accepts a whole slice of a Copy type and uses ptr::copy_nonoverlapping instead, and I got a 5% speedup in my application. I can open a PR if there is interest.

[Manishearth]

That seems useful, yes