Replace directory labels with prefixes

[bugdea1er]

The string_view argument to the directory constructor no longer serves as a parent name to the temporary directory; instead, it is now used as a prefix to the directory name

This is done because:

• This is closer to the mktemp documentation examples, which create tmp.XXXXXX paths rather than tmp/XXXXXX
• If one user program created a directory with a parent label, then another user program would sometimes fail to create a directory with the same label

Also, the parent directory for temporary directories is no longer forced to be created. The C++ standard requires std::filesystem::temp_directory_path() to return an existing directory; if it is deleted after getting the path and before creating our directory, we choose to fail as soon as possible