memoize smartcalc operation execution

[superhawk610]

Currently, SmartCalc::execute is run every 250ms, regardless of whether any input is available or if the input has changed since the last time it was run. Ideally, execute should only be called when there is new/changed input. Some amount of caching may also be desirable.

~~This is currently all done inline inside spawn, which also isn't ideal. It should probably be moved into its own module.~~ Done in #9.