feat: add wrenGetValueDouble API and benchmark for wrenCall() performance

Add a new public API function wrenGetValueDouble() to extract numeric values from WrenValue handles, with proper assertions for NULL and type checking. Introduce a comprehensive benchmark test for wrenCall() that measures invocation overhead by calling a foreign method 1,000,000 times across a separate VM instance, validating result correctness and reporting elapsed time. Include the benchmark script api_call.wren and register it in the Python benchmarking harness. Also fix minor assertion message typos in wren_vm.c for consistency.

src/include/wren.h

@@ -226,6 +226,12 @@ WrenInterpretResult wrenCallVarArgs(WrenVM* vm, WrenValue* method,
                                     WrenValue** returnValue,
                                     const char* argTypes, va_list args);
 
+// Gets the numeric value of [value].
+//
+// It is an error to call this if the value is not a number.
+double wrenGetValueDouble(WrenVM* vm, WrenValue* value);
+// TODO: Functions for other types.
+
 // Releases the reference stored in [value]. After calling this, [value] can no
 // longer be used.
 void wrenReleaseValue(WrenVM* vm, WrenValue* value);

src/vm/wren_vm.c

@@ -1474,9 +1474,17 @@ WrenValue* wrenCaptureValue(WrenVM* vm, Value value)
   return wrappedValue;
 }
 
+double wrenGetValueDouble(WrenVM* vm, WrenValue* value)
+{
+  ASSERT(value != NULL, "Value cannot be NULL.");
+  ASSERT(IS_NUM(value->value), "Value must be a number.");
+  
+  return AS_NUM(value->value);
+}
+
 void wrenReleaseValue(WrenVM* vm, WrenValue* value)
 {
-  ASSERT(value != NULL, "NULL value.");
+  ASSERT(value != NULL, "Value cannot be NULL.");
 
   // Update the VM's head pointer if we're releasing the first handle.
   if (vm->valueHandles == value) vm->valueHandles = value->next;
@@ -1734,7 +1742,7 @@ void wrenSetSlotBool(WrenVM* vm, int slot, bool value)
 
 void wrenSetSlotBytes(WrenVM* vm, int slot, const char* bytes, int length)
 {
-  ASSERT(bytes != NULL, "Byte arraybytes cannot be NULL.");
+  ASSERT(bytes != NULL, "Byte array cannot be NULL.");
   setSlot(vm, slot, wrenNewString(vm, bytes, (size_t)length));
 }