Improve object.js a·p·i

- Add tests for functions which were formerly just re·exports of
  standard library functions.

- Test constructibility, name, and length for all exported functions.

- Replace `isFrozenObject` and `isSealedObject` with `isUnfrozenObject`
  and `isUnsealedObject`, as the former versions return _true_, not
  _false_, when provided with a non·object argument.

- Revise `setPrototype` to have better behaviours than the Ecmascript
  implementation when provided with a non·object argument.