Split the bulk of the code in LcovReporter.lcov_file out into two free helper
functions, lcov_lines and lcov_arcs.  This is easier to read and will make it
easier to do future planned changes in a type-safe manner.

No functional changes in this commit.

The branch field of a BRDA: record can be an arbitrary textual label.
Therefore, instead of emitting meaningless numbers, emit the string
“to line <N>” for ordinary branches (where <N> is the arc destination
line, and “to exit” for branches that exit the function.  When there is
more than one exit arc from a single line, provide the negated arc
destination as a disambiguator.

Thanks to Henry Cox (@henry2cox), one of the LCOV maintainers, for
clarifying the semantics of BRDA: records for us.

Quite straightforward: a function has been executed if any of its region’s
lines have been executed.

Should fix the test failures with pypy pretending to be python 3.8.

There is a bug somewhere, in which if we collect data in --branch mode under
PyPy 3.8, regions for top-level functions come out of the analysis engine with
empty lines arrays.  The previous commit prevented this from crashing the lcov
reporter; this commit adjusts the tests of the lcov reporter so that we expect
the function records affected by the bug to be missing.

I don’t think it’s worth trying to pin down the cause of the bug, since Python
3.8 is approaching end-of-life for both CPython and PyPy.

.