* spot/tl/formula.hh (formula::operator bool): Make it explicit.  When
compiling in C++20 mode with g++ 10.1, this bool operator was used
instead of the comparison operators while looking up a
std::pair<formula,formula> in a hash map, causing many test suite
failures.  This problem does not occur with clang++ 10.0, so it
might just be a bug in g++ 10.1.  But having explicit operator bool
is good practice anyway.