Add left recursion support for rules without args

[fgasperij]

Adding a straightforward implementation of the packrat memoization to support left recursion in rules without args.

These changes would resolve #262.

Any feedback is welcome, let me know what you think!

[kevinmehall]

Nice!

I'm thinking this should be a different attribute, maybe #[cache_left_rec] or #[cache(recursive)], rather than changing the behavior of all #[cache]. The overhead of this for a rule that does not use recursion is that it will parse the rule a second time, and always find the same end position and exit the loop. I'm not sure that double-parsing is acceptable, given than #[cache] is most likely applied to rules that have nontrivial parse time when you want to avoid repeatedly parsing them.

[fgasperij]

Yes, completely agree, making all #[cache] users pay that extra call makes no sense, now it's #[cache_left_rec]. Should we change the left recursion error message to say something like If you want to use left recursion, add #[cache_left_rec], but bear in mind that it will be a little more costly.

[kevinmehall]

Should we change the left recursion error message

I'm not sure the error message is the place to go into a ton of detail about the tradeoffs, but there should probably be a section in the documentation describing the different cache modes, and maybe a comparison of the approaches for parsing nested expressions. I'm happy to write this if you'd rather just get this merged.

[fgasperij]

Yes, if we can merge this one, and add the doc in a different PR that would be great. Let me give it a go, I'll put up a PR tomorrow.