C++20 format function clashes with testlib's format

[quangloc99]

The new C++20 format is clashing with testlib's format one. These 2 are not compatible with each other, and they clash each other when there is using namespace std. I'm used to the testlib one when preparing the contest. However, the change in C++20 caught me off-guard, since std::format got resolved by gcc. To use testlib's format, I need to specify ::format.

I think there are 2 things that can be done. One is to warn about the new function, and two is to rename the function/scope it into a different namespace. I'm not sure about the warning part, since it seems not straight forward to do so. But this is a breaking part for a frequently used function.