static_http_cache, a local disk cache for static HTTP resources

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TODO

• proper error reporting
• record usage counts and dates for entries in the cache, so we can automatically clean them up.
• make sure each public type's interface is defined by a trait.
• Cache::get() needs a callback to report download progress.
• if Cache::get() updates the locally cached data, it should delete the file containing the stale data.
• Add support for other caching-relevant headers, like Expires or Cache-Control.
• Support "freshness", so we can sometimes answer from the cache without having to talk to the remote server at all.
• Add support for limiting cache size, so we automatically remove old data if the cache gets too big.