add google's conformance test suite to find bugs

[timotheecour]

just saw it on https://forum.nim-lang.org/t/3687/1#23146 as posted by @oswjk

As a side note: my library currently passes googles conformance test suite for proto3 tests that do not require JSON input/output. A lot of silly mistakes were fixed while running test suite. It's very nice of google to provide it.

looks like should be easy to add, as done here: https://github.com/oswjk/protobuf-nim/blob/master/tests/conformance/

[PMunch]

Currently this implementation supports neither maps nor options, so that would fail at the parsing step. I am planning to add that in the near future though, but until then this will have to wait.

[oskaritimperi]

If you have any questions about the conformance test stuff, just ask. It was very nice to get it running to see how well my creation fares. :-)

When you start implementing it for your library, you can copy-paste as much code as you want from my version (as if there was a lot of it ;-))