Diagnostics: check for frozen objects when attempting `oset`

[darwin]

Unexpectedly dealing with frozen objects could cause a lot of confusion. We should check for frozen objects in dev mode and give user a friendly, descriptive warning.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/isFrozen

[donmullen]

Also Object.seal
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/seal