Add `filterStatic` (vs. `filter`) to ConstraintStreams

[larsbeck]

We often write statements like: factory.forEach(Shift.class).filter(someCondition) where someCondition only uses data that never changes (static).
Whenever a shift changes in the example above, the filter is evaluated. Running this evaluation often (over and over) is costly.

Instead of evaluating the filter every time a shift changes, I propose to cache the filtered list of shifts.

We have worked around this issue by introducing a new type, for which we precompute the filtering and use that instead of the original Shift.class.

Another positiv effect of providing something like filterStatic and describing the difference to filter may be that it may make the behavior of filter even more clear.