Collatz conjecture

Every positive integer > 1 in the form, 2^n * o + 1 for some o odd.
One can then recursively decompose that odd, until o = 1.

In this way, every odd can be uniquely represented as a sequence of
natural numbers.

For example:
13 can be represented as the sequence [2, 1], since 2^2 * (2^1 + 1) + 1 = 13

Assume then that there exists a cycle. There must then be the smallest odd in
the cycle. Now using the previous notation, we can show that certain sequences
will cause a contradiction that there is a smaller odd in the sequence.

TODO:

* Add constructor to CollatzNode class
* Build CollatzExprs
* Make sure to be able to print CollatzExprs
* Bring in priority queue for CollatzExprs
* Add recursive expansion of CollatzExprs
* Add unit tests
* Output state
* Read in state
* Bring in BigInt