Additional diagnostics checks

As you probably know, Javascript can have frozen or sealed objects. Some objects can also have individual properties defined as non-writable. Under dev mode, you probably want to get some warning when trying to oset! via such objects/properties.

We newly check for frozen/sealed objects when trying punch to through them with oset!. Also we newly check for non-writable properties and report such invalid access accordingly. This was requested in #16.