If you've used Moose's native attribute traits, or MooX::HandlesVia
before, you should have a fairly good idea what this does.

Why re-invent the wheel? Well, this is an implementation that should work
okay with Moo, Moose, Mouse, and any other OO toolkit you throw at it. One
ring to rule them all, so to speak.

Also, unlike MooX::HandlesVia, it honours type constraints, plus it
doesn't have the limitation that it can't mutate non-reference values.