Do some work on wrenCall().

- Add an explicit va_list version. That lets variadic functions
  forward to it.
- Fix a GC bug in wrenCall() with return values.
- Make the call API test not re-enter the VM.

test/api/call.h

@@ -1,3 +1,3 @@
 #include "wren.h"
 
-WrenForeignMethodFn callBindMethod(const char* signature);
+void callRunTests(WrenVM* vm);