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/parser/mod.rs

@@ -27,7 +27,7 @@ fn match_span(s: &Span, v: &Value) -> Result<()> {
     match &v {
         Value::String(vs) => {
             my_assert_eq!(
-                *s.text(),
+                s.text(),
                 vs.as_ref(),
                 "{}",
                 s.source