`Twine`s are very cool but very subtle objects. You basically have to
use them as `const Twine &` arguments only. In fact, a Twine keeps a
reference to objects whose lifetime corresponds to the current
statament. Therefore, a `Twine` can easily end up holding a reference to
invalid objects.

This commit fixes a bug in function isolation (which would show up only
if optimizations are enabled) due to a misusage of
`Twine`. Additionally, it reduces its usage to safe places in
`statistics.h`. Finally, we added an assertion in `check-conventions.sh`
to ensure that no variables of type `Twine` are ever declared.