Hello,

First I want to thank you for making this awsome package.
I noticed an issue and this is only my personal assumption FYI.

btree.Set[type] type can be defined as 8 byte int64 or uint64.
In extreme situation, it can storage as much as 18446744073709551615 numbers into it, starts from 0, steps 1.
But for method Set.GetAt(index int), index is an int, also

if tr.root == nil || index < 0 || index >= tr.count {
	return tr.empty.key, tr.empty.value, false
}

On 32-bit machine, the maximum index value is 2147483647, not enough for above situation.
Do you consider change it into an interface{} that includes all Number types?

Thank you very much
Best regards