Caching view function

[raimohanska]

Map/view functions in Observable libraries do not guarantee running the mapping function only on actual input changes.

The issue is illustrated in the Lonna tests: https://github.com/raimohanska/lonna/blob/master/src/view.test.ts#L63

Guaranteeing a minimal number of calls to the function would be very useful in Harmaja, where the functions create actual DOM nodes. It's worth noticing that the components are activated only once, but still, creation of DOM nodes is not something to do just for fun.

Ways to solve this:

• Introduce a Harmaja.view function that caches values (in componentScope?) to avoid creating garbage
• Investigate whether Lonna can be optimized to avoid these calls without sacrificing other good qualities. This may not be generally possible because delegating get() to parent observables is generally a good design because it guarantees freshness