Fix yecc grammar to properly handle the 'packed' option

[tyler-eon]

trafficstars

Currently the "packed" option always throws an error when used as it expects a non-reserved atom value, but should only ever be set using the "true" or "false" reserved words.

[jeremyong]

Yea I understand the error now. We can actually probably delete the line before that since it shouldn't accept arbitrary atoms. Accepting true and false should be sufficient.

[tyler-eon]

Updated; removed the general "atom" line and only use true and false now.

[jeremyong]

Before I merge this, I just remembered that I never got around to implementing packed fields since it was a feature I didn't use haha.

[jeremyong]

Maybe I should leave this open until that happens?

[tyler-eon]

Right now a proto message will raise a syntax error if it includes the packed option. If we want to leave it like that, because there is no formal handling of packed fields, then we can definitely leave this issue open until we can implement packed fields.

Also, facepalm. Haha.