Solved problem 14

This brute force algorithm only takes 3.3s on my machine to run, so
there's really no need to optimze this.

But one possible way to optimize this solution is to store the collatz
sequences of previously calculated starting values into a cache. Then
instead of calculating the complete sequence, we can check the cache,
and only calculate and store part of the sequence to get the total
length.

For example, for the sequence 13 > 40 > 20 > 10 > 5 > 16 > 8 > 4 > 2 > 1
and in the cache, we stored the previously calculated length for
starting values 20, 10, 5, 16, 8, 4, 2, 1. To calculate this sequence,
we only need to calculate from 13 to 40, to 20, but since we know the
length of the sequence from 20 onwards, we can read that from the cache.

Maybe I'll revisit this to implement that optimization, but again,
there's really no need if the brute-force algorithm takes only 3
seconds.