Enums use varint, so if the value is negative it should end up being longer.

This was caught my new conformance test cases.

* Test singular fields are encoded in canonical way

* Defautl values in proto3 should not be encoded.
* Values should be converted to the canonical representation (e.g.,
long int64 value may be truncated for int32 field)

* Update failure list

* Update failure list

* Add conformance tests for explicit packed/unpacked fields

* Fix typo

* Update failure lists

* Update failure list

* Use enum class to make enum scoped

* Add binary conformance test for default repeated fields

1) Both packed and unpacked encoding should be accepted for parsing.
2) Encode should follow the default way for the syntax.

* Uncomment test

* Remove is_primitive

* Add failed tests to failure lists.

* Add failed test to failure list

* Use binary format to specify expected value

Text format cannot distinguish whether repeated field is packed or not.

* Change method name from ToHexString to ToOctString

* Add failed test to failure list

* Add failed test to php's failure list

* Fix comments

If a message is proto3, then the zero values still count as being set one the
field is in a oneof.

Add tests to confirm oneofs work as expected in both syntaxes.

If a message is proto3, then the zero values still count as being set one the
field is in a oneof.

Add tests to confirm oneofs work as expected in both syntaxes.

* Add conformance test for zero fields in oneofs.
* Add failures to the "expected" files.

- Update all the new runner cases to handle a skip result as a skip and not a failure.
- Make ObjC use the new serialize_error result if it fails to generate the data.

[skip ci]

- Mark all conformance tests as now passing.