Bytes: Use ManuallyDrop instead of mem::forget

[braddunbar]

Note: The tsan job is failing in CI due to https://github.com/rust-lang/rust/issues/122476, which should be resolved in the next few days.

Much like https://github.com/tokio-rs/bytes/pull/675, I found a few places we can more idiomatically use ManuallyDrop.