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/interpreter/cases/data/tests.yaml

@@ -37,7 +37,7 @@ cases:
         - 100
         - 200
         
-  - note: overriding refs in data produces error
+  - note: overriding refs in data produces no error
     data:
       test:
         rule1: 0
@@ -46,8 +46,8 @@ cases:
         package test
 
         rule1 = 6
-    query: data.test
-    error: value for rule has already been specified
+    query: data.test.rule1
+    want_result: 0
       
   - note: rule named data
     data: