formula: make operator bool explicit
* 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.