More OPA conformance (#77)

- object.union
- object.union_n
- treat negative integers as two separate tokens (Sub and Number)
  when seen in arithmetic expressions
- Ensure that fully query string is parsed
- Evaluate queries in a separate module instead of the last read module.
  This correctly handles queries of the form `x = data.test.y` where x is
  already a ref in `data.test`
- Handle queries producing multiple outputs in test infrastructure
- Add tests for engine
- Add tests locking down valid queries
- Update opa.passing

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

tests/interpreter/cases/arithmetic/tests.yaml

@@ -0,0 +1,21 @@
+# Copyright (c) Microsoft Corporation.
+# Licensed under the MIT License.
+cases:
+  - note: negative-integer-literal-in-arithmetic-expressions
+    data: {}
+    modules:
+      - |
+        # In the following, the negative integer must be broken into a - and an integer tokens
+        # when in arighmetic expression contexts
+        package test
+        a = 1+1-1
+        b = 1 +1 -1
+        c = 1 + 1 - 1
+        d = -1 -1
+    query: data.test
+    want_result:
+      a: 1
+      b: 1
+      c: 1
+      d: -2
+