Add header + Remove unsafe usage

[Shadlock0133]

I've added header to output, so the columns are hopefully more obvious:

  Lines Copies  Function name
   3062    173  core::ptr::real_drop_in_place
    966      2  alloc::slice::merge_sort
    812      2  alloc::raw_vec::RawVec<T,A>::reserve_internal
    612      2  alloc::slice::insert_head
    586      1  cargo_llvm_lines::read_llvm_ir

I've also noticed one unsafe function, which I don't understand why is it needed. I replaced it with hopefully correct, safe code. I tested it only on Windows, so if it's wrong/breaks on Linux, I can remove the commit.

Edit: Forgot about rustfmt.

[dtolnay]

Thanks!

The create_stdios code might be from before child streams were usable as Stdio (rust-lang/rust#42133).