Add property-based tests and fix the bugs discovered.

Change-Id: I8deb1a7d75078c037714541d8f6f656052c2476c
diff --git a/jgvariant-core/pom.xml b/jgvariant-core/pom.xml
index b37b65c..da34551 100644
--- a/jgvariant-core/pom.xml
+++ b/jgvariant-core/pom.xml
@@ -61,6 +61,11 @@
       <artifactId>junit-jupiter-api</artifactId>
       <scope>test</scope>
     </dependency>
+    <dependency>
+      <groupId>net.jqwik</groupId>
+      <artifactId>jqwik</artifactId>
+      <scope>test</scope>
+    </dependency>
   </dependencies>
 
 </project>