After some ongoing discussions with the Storage API team, this PR
improves support for proto2/proto3 syntax in protocol buffer code.

* Updates testdata so that we have a proto2 and proto3 form of
  our SimpleMessage data.

* Add reference schemas to testdata.

* Updates proto conversion code in the adapt package so it's creating
  proto2 messages by default.  The code paths for doing proto3
  conversions are present, but not exported yet as the storage API
  doesn't properly handle proto3 expectations for conversion. Namely,
  conversion doesn't properly account for default values and use of
  wrapper types.

* Adds benchmarks for dynamic schema generation and static
  serialization, to aid in some internal discussions.