ArenaAllocator.free and Nested Arenas

What happens when you free with an ArenaAllocator? You might be tempted to look at the documentation for std.mem.Allocator.free which says "Free an array allocated with alloc". But this is the one thing we're sure it won't do. In its current implementation, calling free usually does nothing: the freed memory isn't made available for subsequent allocations by the arena, and it certainly isn't…