Ensure that scopes are cleaned up correctly upon error. (#20)

When evaluating rules, upon error the last pushed scope wasn't being
popped from the stack of scopes. This causes incorrect behavior
when there are multiple definitions for the same rule name.

The fix is to make sure that the scopes are popped manually upon encountering errors.

Once the interpreter logic is locked down, then we need to clean up scope management
using Drop functions so that the cleanup happens even during short circuited return.

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

tests/interpreter/cases/call/or.yaml

@@ -130,10 +130,8 @@ cases:
         a2 = fcn(5)
     query: data.test
     want_result:
-      a1: 
-        set!: [hello world]
-      a2: 
-        set!: [6]
+      a1: "hello world"
+      a2: 6
 
   - note: or-all-error
     data: {}
@@ -151,4 +149,4 @@ cases:
 
         a1 = fcn("world")
     query: data.test
-    error: "`add` expects numeric argument. Got `\"world\"` instead"
+    error: "`add` expects numeric argument. Got `\"world\"` instead"