Add functions to return standard directories

[tgross35]

Add the following builtin functions:

• cache_directory()
• config_directory()
• config_local_directory()
• data_directory()
• data_local_directory()
• executable_directory()
• home_directory()

These use the dirs crate to return XDG-compliant paths, or the system defaults.

Fixes: #1820

[laniakea64]

Would it be better to use dirs::preference_dir for config_directory()?

[tgross35]

Would it be better to use dirs::preference_dir for config_directory()?

I think it would be better to keep names consistent with what they are on dirs, which could mean adding preference_directory. But the only difference between the two is on mac - /Users/Alice/Library/Application Support(config_dir) contains a directory with the application name and then whatever contents is needed, pretty in line with other platforms.

/Users/Alice/Library/Preferences (preference_dir) only contains com.*.*.plist files, which are a something like a binary JSON. I think less likely to be used than the config directory where you can just place files.

[casey]

Awesome! Thanks so much for the docs and tests, a lot of PRs miss those. See review for comments.

[tgross35]

I see you changed this to return an error if the directory is not found (thanks for the updates!). Would it be better to make these accept either 0 or 1 argument, so a fallback can be provided if not available?

[tgross35]

Or looks like it's ready to merge. If the above sounds reasonable, I can submit a followup PR :)

[casey]

Nice! I tweaked the readme a little, and changed the functions to return an error if the directory isn't found. Also, for some reason, on my mac the executable directory wasn't found, so I modified the test to handle this error.

[casey]

I see you changed this to return an error if the directory is not found (thanks for the updates!). Would it be better to make these accept either 0 or 1 argument, so a fallback can be provided if not available?

Whoops, I just noticed this comment. If people want it, I think it would be fine to modify these functions to accept a fallback which is returned if the directory isn't found. We could either wait and see if anyone complains, or if you feel like it, and do it in a follow up PR.

[tgross35]

Sounds great. Thanks!