feat(evaluator): implement walk builtin - #187

Go-OPA flags walk as its only relational builtin (Relation: true): it
yields many [path, value] tuples through an iterator instead of
returning a single value. The IR planner lowers it to a plain call plus
a scan over the result, so this returns the whole set of pairs. Sets
are keyed by their own members.

Also fixes composite object keys in opa-jackson, which used Java's
collection formatting instead of compact JSON.

Fixes open-policy-agent#141

Signed-off-by: Sebastian Spaink <[email protected]>