Ability to run the OPA testsuite (#39)

`OPA_TESTS_DIR=path/to/testsuite cargo test opa -- --shot-output` to run opa tests.
Failing test cases are saved in target/opa/folder for investigation.

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

src/parser.rs

@@ -333,7 +333,7 @@ impl<'source> Parser<'source> {
         }
 
         // It could be a set, object or object comprehension.
-        // In all the cases, the first expressoin must parse successfully.
+        // In all the cases, the first expression must parse successfully.
         if *self.tok.1.text() == "}" {
             self.next_token()?;
             span.end = self.end;