Add TempDirCache to cache TempDir results, fixes #13

[twpayne]

This is an alternative solution to #13. Note that it shares some code (getDev) with #14 but addresses separate functionality.

This PR adds TempDirCache, which is an intelligent cache for TempDir. Temporary directories can be re-used for all destination directories on the same device.

It's not clear how to write an automated test for this, as you'd need either to mount a different filesystem (which the user running the test might not have privileges to do) or mock out os.Stat (hooks to do this aren't in place).

Instead, I've included internal/tempdircache_test which exercises the code. See the comment in internal/tempdircache_test/main.go for details, but you can see the cache in action on modern Linux systems with:

$ go run ./internal/tempdircache_test $XDG_RUNTIME_DIR $XDG_RUNTIME_DIR/systemd

[twpayne]

Closing for now following discussion in #13.