sl2: Use object identity and count in slfor.
sl2: Do not pass keywords intended for a displayable to its parent.

Due to inheritance of SLBlocks, we had the problem that

    frame:
        textbutton "test" xysize (100, 100)

Would pass the xysize parameter to both the textbutton and the frame.
sl2: Implement constant displayables.

These are displayables that have constant arguments and constant
children, and hence where the same displayable can alwaye be used.  We
use a very fast path for these displayables (simply retrieving them
from the cache), and avoid evaluating their children entirely.