This migrates to using protobuf to
define the "report" and "changes"
formats in objdiff-cli.
The JSON output now uses the Proto3
"JSON Mapping", which is slightly
incompatible with the existing JSON
format. Mainly, 64-bit numbers are
represented as strings, and addresses
are decimal strings instead of hex.
However, the older JSON format is
still accepted by "report changes"
to ease migration.
