Requiring goal to contain only one statement.

Previously, the parser read the first statement of the goal as its
precondition, but didn’t check that it was properly terminated with a
closing parenthesis. This allowed arbitrary text to be included within
the goal description without error, which was incorrect.

This commit fixes this issue and adds a corresponding unit test.

tests/data/issues/issue-11.pddl

@@ -0,0 +1,15 @@
+; tests that “imply” statements in preconditions are correctly reduced
+(define (domain test-normalization)
+	(:predicates
+		(test-predicate-0))
+)
+
+(define (problem test-normalization)
+	(:domain test-normalization)
+
+	(:init
+		(test-predicate-0))
+
+	(:goal
+		(test-predicate-0)
+		(error)))