Make branch coverage accessible via the API

[eleanorjboyd]

fixes #1888

one edit from above proposal is I exposed branch_stats from the Analysis class as it has the (total_exits, taken_exits) which is the info needed and is already public.

another note is Im not entirely sure how to test / view the changes to the .rst files for https://coverage.readthedocs.io/ so any advice here would be appreciated. Thanks!

[nedbat]

@eleanorjboyd Sorry I haven't gotten to this. I'm pondering whether there's anything in Analysis I wouldn't want to make public. I can take care of checking the .rst changes.

[nedbat]

Commit 33f12df has Coverage.branch_stats().

[eleanorjboyd]

Hi @nedbat- super excited thank you! I blocked off time this upcoming month to implement this- so hopefully I can get it in quick! Will keep you in the loop as I implement and release!