Introduce a new algorithm, regexp::properties::LanguageIsEpsilon to
check whether a language of a regexp is exactly equal to a language of
epsilon regexp.
This will be later useful in checking whether the language of a regexp
is infinite.

Similarly to previous commit, let's make this clear. We are soon going
to add a new algorithm that will check if the language is equal to
epsilon, so let's differentiate.

The name regexp::properties::RegExpEmpty is not that expressive. Let's
make clear what is going on.

and also make sure that the unique terminating symbol is unique among both
subject and pattern