Refactor various filesystem related operations to use std::filesystem

[nightlark]

This change makes use of the std::filesystem API introduced in C++17, and replaces a few instances where variable length arrays and snprintf were being used to construct temporary file paths with std::string (several other places were already using std::string). Using std::filesystem should be a bit more portable than the POSIX function calls that were previously used, which could help get a working native/MinGW build on Windows.

• Remove #include <sys/stat.h> from files where it is no longer needed
• Use std::filesystem functions to check free disk space available, create directories, if a path exists, if a path is a directory, and file sizes
• Use std::filesystem::remove instead of unlink (behavior on POSIX should be identical)
• Use std::string data() method to get a mutable char *, instead of casting away the const-ness of c_str() (since C++11, the array returned by data() is null-terminated)
• Replace use of VLAs for temporary file path construction with std::string
• Replace hard-coded use of /tmp for temporary directory with call to std::filesystem -- on Linux/macOS this returns /tmp by default unless the user sets an environment variable to override it (TMPDIR, TMP, TEMP, TEMPDIR), and on Windows this returns the same thing as GetTempPath

[nightlark]

Any reviewers have questions, feedback or requested changes (or just haven't had time to review yet)?

Also happy to set up a zoom/webex/teams call to go through the various changes if that would help.

[nightlark]

@e-n-f any feedback on these changes?

[nightlark]

Hi! It's been a year, any thoughts on this @e-n-f, @dnomadb, @ChrisLoer, @arredond, or @migurski?