Make chpl_library_finalize re-entrant

We were encountering a problem when chpl_library_finalize was getting called
multiple times - exiting the Python program would result in:

```
Assertion failed: (data && data->bundle), function chpl_task_getInfoChapel, file chpl-tasks-impl-fns.h, line 140.
```

because our generated `__init__.py` file added an `atexit` call to clean up the
runtime, but code could invoke an explicit cleanup call as well.

I need to take a look if we explicitly state you need to make that cleanup call,
but this may also help in cases where multiple Chapel libraries are used in a
Python program.

----
Signed-off-by: Lydia Duncan <[email protected]>

runtime/src/chpl-init.c

@@ -441,13 +441,23 @@ void chpl_library_init(int argc, char* argv[]) {
 // we may have initialized
 extern void chpl_deinitModules(void);
 
+bool lib_deinited = false;
+
 //
 // A wrapper around chplexit.c:chpl_finalize(...), sole purpose is
 // to provide a "chpl_library_*" interface for the Chapel "library-user".
 void chpl_library_finalize(void) {
   if (!lib_inited) {
     return;
   }
+
+  if (lib_deinited) {
+    // Nothing to do, we've already been cleaned up
+    return;
+  } else {
+    lib_deinited = true;
+  }
+
   chpl_libraryModuleLevelCleanup();
   chpl_deinitModules();
   chpl_finalize(0, 1);