Replace `std::string` with `std::filesystem::path` for file paths

[ssoelvsten]

There are a few places where we use std::string as a file path. We should replace these with std::filesystem::path to convey intent and make use of its features.

Here is a list of all places that need changing.

• adiar.h
  • adiar_init(...)
• dot.h
  • __print_dot(...)
  • print_dot(dd_t, ...)
  • print_dot(shared_levelized_file<arc>, ...)
• bdd.h
  • bdd_printdot(...)
• zdd.h
  • zdd_printdot(...)
• internal/io/file.h
  • file::file(path)
  • file::path(path)
  • file::set_path(path)
  • file::move(path)
• internal/io/levelized_file.h
  • levelized_file::canonical_file_path(path_prefix, idx)
  • levelized_file::canonical_levels_path(path_prefix)
  • levelized_file(path_prefix)
  • levelized_file::make_persistent(path_prefix)
  • levelized_file::paths()
  • levelized_file::move(path_prefix)
• internal/io/shared_file_ptr.h
  • shared_file_ptr(p)
  • make_shared_file(p)
  • make_shared_levelized_file(p)