Create framework for file-level cache

[idle-code]

Tags might perform expensive operations like reading metadata blocks from a large file or accessing the network; when used multiple times in the same template, those operations might be repeated unnecessarily - for example when extracting artist name and track title simultaneously.
To combat this, a short-lived cache can be utilized. Fortunately, there is already file descriptor caching in place on the File class, so the cache can also be placed there.