pack/ee/ee.go: packdata invariants
pack/ee/ee.go: packdata invariants

Adopt Rob's proposed packdata invariants.

Change-Id: I4578b3b0e8e72219f7d774ae9d3e1719242f4f7f
Reviewed-on: https://upspin-review.git.corp.google.com/1302
Reviewed-by: Eduardo Pinheiro <55040cbb39ba395339ae7940d8da6cd97797a4a8@google.com>
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@google.com>

upspin/upspin,upspin/upspin