Create a log of the grammar paths take an attach as metadata to top of result

[kanaka]

This proposed change keeps track of all the paths take through the grammar during parsing. I'm using instaparse with this change to analyze a corpus of test cases and then use test.check to generate new tests cases that are probabilistically "similar" to the corpus. I use the path log to create a weight file for my test.check generators.

I don't necessarily consider this in final form but it's currently working for me so I'm creating this PR to get feedback/discussion.

I know that performance behavior of instaparse is critical. Do you have performance tests that I could use to test this change to characterize what impact if any it has on performance?