Ability to evaluate rules directly instead of evaluating queries

[anakrish]

OPA queries are similar to rule bodies or comprehension bodies. They can contain multiple rego expressions, multiple statements, and even loops. The QueryResults structure returned by evaluating the queries is also verbose - the value produced by each statement is captured along with the location information.

In many cases, such detail is not needed. All that is needed is the ability to execute a given rule and return the output.
Such an API would be faster than eval_query.

Here is another use case: #36 (comment) that favors evaluating rules directly instead of a query.