OPA conformance (#71)

- Remove unnecessary memory allocations
- Add --non-strict flag
- Ensure that only empty modules (ones without rules) are initialzed prior to evaluating rules.
- Record rule as entry for each of its prefixes.
  For example, for a rule a.b.c =... in package test, record it in
  rules["data.test.a"], rules["data.test.a.b"] and rules["data.test.a.b.c"]

  This allows evaluating the correct list of rules based on expessions
  a.b.c, a.b, a, data.test.a.b.c, data.test.a.b, data.test.a

Closes #69
Closes #70

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

tests/opa.rs

@@ -169,7 +169,8 @@ fn run_opa_tests(opa_tests_dir: String, folders: &[String]) -> Result<()> {
                 {
                     entry.0 += 1;
                 }
-                (Err(_), None) if case.want_error.is_some() => {
+                // TODO: Handle tests that specify both want_result and strict_error
+                (Err(_), _) if case.want_error.is_some() => {
                     // Expected failure.
                     entry.0 += 1;
                 }