Add a function to access application data directory

Currently there is only a way to easily store application specific data using the standard library, std::env::home_dir. That's not a quite correct way. I was thinking about the addition of a std::env::appdata_dir function?

The home directory is only suitable for storing application data on Linux (and perhaps other *nixes?), on Windows the correct place is %AppData% or %LocalAppData% and on OSX it seems to be ~/Library/Preferences/ or ~/Library/Application Support/.

That brought my attention when I noticed that Cargo stores it's data on %USERPROFILE%/.cargo/ (std::env::home_dir gives%USERPROFILE on Windows), note how directories starting with . aren't hidden on Windows and those pollute the user profile directory.

However I have a few concerns about this myself:

• Not sure whether std::env should have functions that aren't directly a enviroment variable (e.g. ~/Library/...) though it says and various other important directories on it's description.
• Storing application data might be more complicated than that (especially on OSes not mentioned here) and it'd thus be better to have this kind of feature as a separate user crate.

Start by reviewing std::env::home_dir and the platform-specific locations listed in the issue. Resolve whether an appdata_dir API belongs in std::env or a separate crate, define supported operating-system behavior, and document the accepted design as done.