Add packed roundtrip test
Add packed roundtrip test

SymbiFlow/pycapnp,jparyani/pycapnp,rcrowder/pycapnp,jparyani/pycapnp,SymbiFlow/pycapnp,rcrowder/pycapnp,tempbottle/pycapnp,tempbottle/pycapnp,jparyani/pycapnp,rcrowder/pycapnp,tempbottle/pycapnp,SymbiFlow/pycapnp,jparyani/pycapnp,tempbottle/pycapnp,SymbiFlow/pycapnp,rcrowder/pycapnp