Add some helpers for function caching.

[jakuub]

If we have persistent data and components that efectively count if they should update, cached functions would be very helpful.

The main scenario is something like this:

1. the component get new props
2. it count new props for some childrens with "functional" functions (functions that only count result, not modify anything)
3. put new props for children

It will be nice, if always from same arguments are the same results counted. It means, that children components will benefit from the same persistent data and can optimize shouldUpdate method.
It also "speed up" the counting.