Some of these cases currently don't occur in practice, but are included for
completeness, and to avoid having to add them later as branch coverage and
MC/DC coverage start building more complex expressions.
Some of these tests use non-standard formatting that we can simulate by
strategically adding `//` line comments.
One contains `where` clauses that would be split across multiple lines, which
we can keep on one line by moving the bounds to the generic type instead.
In some cases we need to prepare a coverage expression that is the sum of an
arbitrary number of other terms. This patch simplifies the code paths that
build those sums.
This causes some churn in the mappings, because the previous code was building
its sums in a somewhat idiosyncratic order.
