fix(sermux): OwnedPortChunk leaving port in chunk

[hawkw]

Currently, the behavior of the OwnedPortChunk::decode function is
incorrect. The returned chunk in the OwnedPortChunk is constructed
by allocating a new Vec from the input, decoding a PortChunk from
it, and then shrinking the Vec to the size of the chunk component of
the returned borrowed PortChunk.

This does the wrong thing. Vec::shrink_to shrinks the Vec from the
end, so we are shrinking it down to the first chunk.len() bytes.
The port number is at the beginning of the Vec, rather than the end,
so instead of dropping the port number and keeping the data, we are
dropping the last two bytes of the data and producing a buffer that
contains [port[0], port[1], data[0], ... data[len(data) - 2]]. This
means the data is silently corrupted.

This branch fixes this behavior by instead constructing the owned chunk
using pc.chunk.to_vec(). This has the unfortunate consequence of
meaning that we allocate twice, rather than once, but it means that we
now return the correct data when using OwnedPortChunk. Alternatively,
we could use Vec::remove twice to remove the first two bytes of the
Vec, but that's two O(n) operations, which is probably at least as
bad as allocating. We could, potentially, change the
OwnedPortChunk::decode operation to not just call PortChunk::decode,
but that would be more complicated.

Adding a failing test for this is as simple as adding a round-tripping
test for OwnedPortChunk as well as PortChunk, which we didn't have
previously. I've also changed the round-tripping tests to use proptest
to generate arbitrary port numbers and chunks to round trip,
because...why not?

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