[roll] Roll fuchsia [packet] Relax BufferView lifetime for &mut &[u8]

In our implementation of `BufferView` for `&'b mut &'a [u8]`, we
previously used the more restricted lifetime `'b`. Concretely, we had
the impl:

  impl<'b, 'a: 'b> BufferView<&'b [u8]> for &'b mut &'a [u8]

This is problematic because it means that, if a caller holds a reference
with lifetime `'b` to an object which owns a buffer with the longer
lifetime `'a`, the caller can only use `BufferView` with the shorter
lifetime `'b`. In practice, this has consequences for situations like
parsing a packet from a buffer which is stored in a local variable. If
the buffer's storage has a longer lifetime, we want to be able to parse
and produce a packet object with that same lifetime, but this
`BufferView` impl means that we can only produce a packet object with
the shorter lifetime of the local variable.

We change this impl to use the longer lifetime `'a`:

  impl<'b, 'a: 'b> BufferView<&'a [u8]> for &'b mut &'a [u8]

This solves the problem, and allows us to parse a packet object with a
lifetime which outlives the lifetime of the local variable used to store
the buffer.

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/594026
Original-Revision: 9d59b7ded5f686bc0faf20744564394bd3ddcf01
GitOrigin-RevId: 5c6582c59192abe9d2148f276c86fa8d8ec8021e
Change-Id: I828a20a0448d17ab714a23a3d71788c67466c6b9