Always wrap functions in closures.

This has a couple of pros:

- It simplifies some code that used to have to check whether a called
  thing is a bare function or a closure.
- It's faster because we don't need to do the above checks every time
  something is called.
- It lets us more precisely type some fields that used to be Obj*
  because they could hold an ObjClosure* or ObjFn*.

The cost is that we allocate a closure every time a function is
declared, even if it has no upvalues. Since functions are called way
more often than they are declared, this is still a net win.

On my Mac laptop:

api_call - wren            0.06s 0.0020 104.73%
api_foreign_method - wren  0.32s 0.0040 101.89%
binary_trees - wren        0.23s 0.0057  98.82%
binary_trees_gc - wren     0.79s 0.0170  98.46%
delta_blue - wren          0.13s 0.0031 101.36%
fib - wren                 0.23s 0.0038 103.15%
fibers - wren              0.04s 0.0017  98.97%
for - wren                 0.08s 0.0017 107.81%
method_call - wren         0.12s 0.0024  98.60%
map_numeric - wren         0.31s 0.0052 103.93%
map_string - wren          0.11s 0.0113  97.97%
string_equals - wren       0.20s 0.0023 107.75%

src/optional/wren_opt_meta.c

@@ -12,17 +12,24 @@ void metaCompile(WrenVM* vm)
   // Evaluate the code in the module where the calling function was defined.
   // That's one stack frame back from the top since the top-most frame is the
   // helper eval() method in Meta itself.
-  Value callingFn = OBJ_VAL(vm->fiber->frames[vm->fiber->numFrames - 2].fn);
-  ObjModule* module = IS_FN(callingFn)
-      ? AS_FN(callingFn)->module
-      : AS_CLOSURE(callingFn)->fn->module;
+  ObjClosure* caller = vm->fiber->frames[vm->fiber->numFrames - 2].closure;
+  ObjModule* module = caller->fn->module;
 
   // Compile it.
   ObjFn* fn = wrenCompile(vm, module, wrenGetSlotString(vm, 1), false);
 
   // Return the result. We can't use the public API for this since we have a
   // bare ObjFn.
-  vm->apiStack[0] = fn != NULL ? OBJ_VAL(fn) : NULL_VAL;
+  if (fn == NULL)
+  {
+    vm->apiStack[0] = NULL_VAL;
+  }
+  else
+  {
+    wrenPushRoot(vm, (Obj*)fn);
+    vm->apiStack[0] = OBJ_VAL(wrenNewClosure(vm, fn));
+    wrenPopRoot(vm);
+  }
 }
 
 const char* wrenMetaSource()