Treating std::string as UTF-8 is deprecated in std::filesystem::path
since C++20.
However, it makes this codebase easier to read and maintain to retain
the ability for std::string to hold UTF-8.
All code in this repo uses <util/fs.h>, except for a few lines. This is
confusing and potentially dangerous, if the safe <util/fs.h> wrappers
are not used.
The fs.* files are already part of the libbitcoin_util library. With the
introduction of the fs_helpers.* it makes sense to move fs.* into the
util/ directory as well.
