I found that a crate has one or more versions on my local computer. For example,
the quickcheck has version with 0.4.1, 0.5.0, 0.6.2. All crates occupy
hundreds of MB of my computer. this program is aimed to remove all old versions
and just save the last version of one crate.
Note: some application may specify a specified version of a crate. So, when rerun or rebuild it, there are more versions appired, and rerun this program, reremoved it again.