Simplify allocation of JNI global classes and exceptions (#1110)

Helps ensure they get freed properly (We have had a few cases before where this wasn't the case).

wpiutil/src/main/native/include/wpi/jni_util.h

@@ -71,6 +71,11 @@ class JClass {
   jclass m_cls = nullptr;
 };
 
+struct JClassInit {
+  const char* name;
+  JClass* cls;
+};
+
 template <typename T>
 class JGlobal {
  public:
@@ -618,6 +623,11 @@ class JException : public JClass {
   jmethodID m_constructor = nullptr;
 };
 
+struct JExceptionInit {
+  const char* name;
+  JException* cls;
+};
+
 }  // namespace java
 }  // namespace wpi