Lcov report improvements part 2

[zackw]

• break up lcov_file to not be so gigantic
• improved branch destination reporting
• function coverage!
• addresses #1850 and #1852

Because of the refactor in the first commit, functional changes will be easier to see if you go commit by commit.

[zackw]

I think this is ready to be reviewed now. Let me know if you want me to clean up the commit history or anything.

[nedbat]

BTW, I'm about to drop Python 3.8, so you can simplify the tests by just skipping them on pypy3.8.

[zackw]

Testing whether the existing @xfail_pypy38 decorator does the trick now.

[nedbat]

Looks good, thanks so much! I'll merge it.

[nedbat]

This is now released as part of coverage 7.6.2.