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.wren

@@ -27,11 +27,8 @@ class Api {
     // Otherwise print it.
     System.print(value)
   }
-
-  foreign static runTests()
 }
 
-Api.runTests()
 // expect: noParams
 // expect: zero
 // expect: one 1