replace mem with fast-memoize

[43081j]

mem pulls in a few levels deep of dependencies for something pretty simple.

here i've replaced it with fast-memoize which has no dependencies and seems much faster/simpler.

let me know if you want any changes

FYI the default serializer is JSON.stringify internally so the cache will be keyed by the stringified args afaict (equivalent of before, almost)

[raineorshine]

Looks good! Thank you!