Conformance test suite

[lchenut]

Using the conformance_test_runner from https://github.com/protocolbuffers/protobuf This PR adds several things in order to make the test executable, such as

• oneof, right now it's parsed and skipped, but without this addition it doesn't compile
• added support of enum (using https://github.com/status-im/nim-protobuf-serialization/tree/add-enum-support) so it misses some of the cases we talked about here #43
• added map support, making Table a serializable type
• using reference for recursive object

Currently, there's a lot of tests that fail mainly because of #40 which has not been merged yet (writing all-defaults proto3 object fields creates a lot of error, for example)