Regression on #35

[leotaku]

It seems like, with the v2.0.0-alpha3 version of participle, positions are, again, no longer added to structs that implement the Capture interface. This apparently was first fixed in f14a17d in response to #35, but the associated test has since been removed and the issue has reappeared.

I have also tested that this issue is present on master by adapting and adding the old TestIssue35 into parser_test.go and observing a failed test run.