Add an info subcommand for `cargo clean`

[ehuss]

Problem

Without any insight into what caches exist or how big they are, the user does not have a grasp of what the situation is on their machine, or what cargo is leaving on the disk. The new cargo clean gc command can delete some of this data, but the user doesn't have a clear way to understand what the situation is before running that command, or knowledge of what they might want to delete.

Proposed Solution

Add a subcommand (cargo clean info?) which can provide human-readable information about the status of caches and how much disk space they are using.

Notes

Similar related commands:

• cargo cache query
• npm cache ls
• nuget locals all -list
• pip cache info and pip cache list
• dpkg-query?
• Various docker commands (docker system df, docker system info, docker * ls., etc.)
• brew info