Reorder `Vec` fields

[cgaebel]

Currently, raw::Slice is implemented implemented as len, data, and Vec is implemented as len, cap, data. This means that the following code requires a branch, which is extremely unfortunate for my use-case:

enum MaybeOwnedBuffer<'a> {
  OwnedBuffer(Vec<u8>),
  BorrowedBuffer(raw::Slice<u8>, ContravariantLifetime<'a>),
}

impl<'a> MaybeOwnedBuffer<'a> {
  #[inline(always)]
  unsafe fn as_raw_slice(&self) -> raw::Slice<u8> {
    match *self {
      OwnedBuffer(ref v)       => mem::transmute(v.as_slice()),
      BorrowedBuffer(ref s, _) => *s,
    }
  }
}

Ideally, the order of Vec's fields should be changed to len, data, cap to make conversion free.

Thoughts?

[thestinger]

I think you made a minor typo, as slices are currently (ptr, len). I'll r+ a pull request switching Vec<T> to (ptr, len, cap) instead of (len, cap, ptr).