Rust: Allocation::bytes should be private

[RalfJung]

The bytes field of an Allocation is a very subtle one -- the values in there often do not mean what one might intuitively think. First of all the doc comment should probably be improved, but secondly I think we need to make this field private. See https://github.com/rust-lang/rust/pull/62655#discussion_r306753992 for a very subtle bug that happened because it was not private.

Cc @oli-obk @eddyb

[eddyb]

Might also help to rename it to something like unrelocated_bytes.

[RalfJung]

unrelocated_raw_maybe_initialized_bytes_dont_touch