It looks like makizushi is only caching the alphanumeric markers, but not symbol icons. This could be a significant performance issue since every symbol render requires a file read.

We need better instrumentation in the library anyway, so this would be a good opportunity to emit metrics during maki render.

Also, caching all of the alphanumerics could be wasteful, we should try to get a memory profile of how much space this takes. It would be straightforward to replace the permanent cache with a LRU layer, if it would help.

Code to review: https://github.com/wikimedia/makizushi/pull/5