A while ago, caching was removed due to some issues (see #349).
This is a new implementation with the following improvements:
* Architecture: The new implementation is decoupled from the collector, which was a major issue in the last version. Now the collector has a single responsibility: collecting links. This also avoids race-conditions when running multiple collect_links instances, which probably was an issue before.
* Performance: Uses DashMap under the hood, which was noticeably faster than Mutex<HashMap> in my tests.
* Simplicity: The cache format is a CSV file with two columns: URI and status. I decided to create a new struct called CacheStatus for serialization, because trying to serialize the error kinds in Status turned out to be a bit of a nightmare and at this point I don't think it's worth the pain (and probably isn't idiomatic either).
This is an optional feature. Caching only gets used if the `--cache` flag is set.
