*MappedBigList should implement a FlyweightPrototype interface

[seirl]

Currently there is no way to check dynamically if an BigList implements copy() or not. If all the MappedBigList implemented FlyweightPrototype, it would be easier to make a function that makes a threadsafe copy of any type of BigList.

Unfortunately FlyweightPrototype is in dsiutils, not fastutil, so I don't know what's the best way to proceed here.

[vigna]

Well, it's a one-time thing, so one not using reflection? I realize the copy() might not be lightweight. But which other cases would you need to cover (except Mapped vs. non-Mapped)?