Use unsigned integers when dealing with non-negative values

[liamsi]

Discussion started here #43 (comment) (refactor node.level to an unsigned integer value or panic/err if methods that expect a non-negative int receive a negative on; find out which approach is safer/faster/more reasonable).

Also, clarify if and how we want to deal with overflows (also in case of uint(32/64) if we go with unsigned values).

[liamsi]

An interesting read on the signed/unsigned question: http://www.soundsoftware.ac.uk/c-pitfall-unsigned

[masomel]

Good read! I did some small tests, and it seems like uint32and uint64 have the same underflow issue as described in the article. Given this, I think it might be safer to panic/err if methods that expect a non-negative int receive a negative one.