Add some way of caching generated images

[kutis96]

Caching would significantly speed up regeneration of documents with many figures, as the figures would be reused between runs.

Goals:

• Cache generated figures in a pre-configured directory
• Determine which figures were generated by the same source, and reuse the generated assets
• Caching - Delete old, long unused figures from cache, when configured to do so #39 Add some way of automated cache flushing/deletion of unused figures

Implementation ideas:

• Store the cached files as ${hash of absolute path}_${hash of figure source code}.cache in the cache directory
  • As of Added basic caching, moved most debug-level println's to loggers. #38, files are stored as ${hash of figure source code}.cache only
• Read and parse all such file names into memory on startup, and also the files' modification dates
  • As of Added basic caching, moved most debug-level println's to loggers. #38, files are not listed into memory on startup, but are queried nly when needed
• Lookup and compare by ${hash of figure source code} during a render
  • Currently implemented, as of Added basic caching, moved most debug-level println's to loggers. #38
• If automated cache deletion has been enabled (to be implemented in Caching - Delete old, long unused figures from cache, when configured to do so #39):
  • Delete cached items if they weren't used during the last run, and their TTL hasn't ran out
    • TTL - users may revert back to older versions while they're editing, but it seems unlikely such a thing would happen after some period of development, when the user would focus on other parts of the document. Old development versions of these files may be removed again.
• Default .spacedoc/cache directory should probably be in the directory currently being ran from
• Configurability:
  • Whether to actually use cache at all
  • Default cache directory
  • Whether to ever delete cache items for unused files
  • What maximum TTL should cached items have