Custodial refactor of `NSMenu`

[reactive-firewall]

Context:

The current NSMenu implementation is still rather fragile and could use a custodial refactor to add stability with some sanity checks and performance enhancements.

Namely:

• define behavior for nil-edgecases (e.g., fast returns or return nil without nil-error)
• modernize for NSCoding protocol (including transparent support for sub-class overloading)
• shim support for compiler nullability attributes(future-proofing, and required for ever supporting apple's swift-bridge)
• add FIXME/TODO custodial comments for future contributors (both easy stuff, and todo design stuff)
• modernize internal storage

etc.