using a sized type i.e., int32 or int64 for leaf index in the foundInRange method of nmt

For context please refer to the conversation on this issue https://github.com/celestiaorg/nmt/issues/70

Originally suggested by @elias-orijtech in this [comment](https://github.com/celestiaorg/nmt/issues/70#issuecomment-1549928935)
> If you do settle on a signed type, I suggest being specific about the type (`int32` or `int64`), or make the build fail if compiled on platforms where `int` is not at least 64 bits wide.

Link to the foundInRange method: https://github.com/celestiaorg/nmt/blob/0e219c88a3471351159db7967a149d06aaa6dd50/nmt.go#L435