This change changes `buffer.data` into a `[]byte`, from `[bufferSize]byte`.
In exchange, each `buffer` is now grouped together to reduce the number of
allocation. Plus, `View` now holds an embeded list of `buffer` (via `pool`) to
support the happy path which the number of buffer is small. Expect no extra
allocation for the happy path.
It is to enable the use case for PacketBuffer, which
* each `View` is small (way less than `defaultBufferSize`), and
* needs to dynamically transfer ownership of `[]byte` to `View`.
(to allow gradual migration)
PiperOrigin-RevId: 333197252
Ting-Yu Wang