Optimize rb_namespace_available

Rather than to lazily check the env using a trinary
value, we can more straightforwardly check for the
env during the VM boot.

This allow `rb_namespace_available` to just be a pointer
dereference.

eval.c

@@ -78,6 +78,7 @@ ruby_setup(void)
 #endif
     Init_BareVM();
     rb_vm_encoded_insn_data_table_init();
+    Init_enable_namespace();
     Init_vm_objects();
     Init_fstring_table();