More efficient list encoding for Countable

This changes the encoding used for finite lists of values of countable types to be linear instead of exponential. The encoding works by duplicating bits of each element so that 0 -> 00 and 1 -> 11, and then separating each element with 10. The top 1-bits are not kept since we know a 10 is starting a new element which ends with a 1.

Please don't hold back on feedback. I'm sure this could do with some improvements :)