tor-cell: Refactor relay cells to copy much less

We now manipulate raw relay cell bodies as (an alias for)
`Box<[u8;509]>` rather than as (an alias for) `[u8;509]`.  This
enables us to do much less copying.  It will become more important
soon, as we defer parsing relay cell bodies even longer.

Related to #7.

We also use SliceWriter to avoid allocating a Vec<> for every relay
message we want to encode, and instead encode directly into the
cell.