This uses the capn_deflate function.
The current use may get smaller packing than the maximum possible due to
the inability to peak around segments as it packs each segment on its
own. This saves time compared to copying everything into one place and
saves effort by not requiring to change the interface of capn_deflate to
support multiple different buffers.
It should be possible to make the capn_deflate state machine better to
handle the multiple buffers case too.
