Plan: Ghost std::istream & std::ostream and get close to std::filebuf

[buck-yeh]

In advent of std::print(), I feel obliged to remove direct links to std::ostream & std::istream, especially in UnicodeCvt.h& bux::I_SyncLog `.

The part of migration left to be decided is whether replace them with std::filebuf or an abstract common base.