#48 defer evaluation of top-level params/vars until needed

[Mike Brown]

Top-level variables and parameters are evaluated during
stylesheet priming, even if they are not going to be
used. We do catch the case where the variable is
overridden by one from a stylesheet with higher import
precedence, but not the case where the variable isn't
referenced at all.

It would preferable to postpone evaluation until it is
clear that there is a corresponding variable reference
that is going to need to be evaluated.