This repo should define the grammar of the .lyg files

[ckaran]

I think that the grammar for the .lyg files is defined at https://github.com/rust-lang/gll/blob/master/README.md, but given that this repo is going to become the definition of what rust syntax is, I think it would be better if the grammar for the .lyg files was fully specified within this repository. Having to guess as to what a formal grammar's actual definition is just feels bad...