Support annotating return types next to rules

[lolbinarycat]

the proposed syntax would be:

RuleName TYPES <- "grammar"

where TYPES is anything that is a valid return type for a go function. e.g. (string,error), int16 or even nothing (like in func main()).

This return type would then be copied exactly to the generated code.

[breml]

@lolbinarycat Thanks for your proposal and sorry for my slow answer. I looked into this idea some time ago. It turned out to be more complicated than I initially thought. But feel free to start with an implementation and open a PR, then we can work on this.

[lolbinarycat]

@breml I originally planned to do just that and open a PR, but I've done a fair bit of work on various parsers, and I'm a bit tired of it for now. I may get back to it sometime though.

[breml]

@lolbinarycat Never mind, I leave the issue open for now.

[breml]

Duplicate of #62