topdown: evaluate "every"

This includes simple evaluation and partial eval.

For PE, we're deliberately unsophisticated for the first iteration:
if anything is unknown, we'll save the expression.

Due to some technicality, the saved expression's body will be less
clean than usual: copy-propagation is not yet applied.

Besides fixing that, future improvements could unroll the every
expression when the domain is known; or do more work to PE the
expression body when.

Known TODOs: tracing is not yet in the desired state.

There are added test cases in the YAML test case files, and in
topdown_test.go: the tests defined in topdown/topdown_test.go allow
for finer-grained assertions, using `print()` to capture the
evaluation.

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>