Issue title: Any way to see the code coverage of code using named!() ?
 Issue body: It seems that with [runtime-macros](https://crates.io/crates/runtime-macros) we can have unit test coverage of macros but I'm not sure if it can be used with nom.
 Comments: 
 Comment 0: coverage of nom parsers defined with named can be done by calling the parsers in your tests. That's how I do coverage for nom combinators
{'source': 'GithubIssues', 'repo_name_with_owner': 'Geal/nom', 'issue_id': 388795891, 'repo_stars': 7332.0}