fix bugs in the write-path affecting empty structs and List(Text)
for robustness, use version-handling code in writePtr between segments
add versioning test structs to aircraft.capnp schema
use the version-handling code everywhere, and special case Text and Data copies for speed.
note empty structs specifically
first version test in version_test.go passes
version tests on data passing, pre-cleanup
all tests green. new version handling code and perf improvements in place.