Countable instance for vec, and rename `vec_to_list_of_list` into `vec_to_list_to_vec`

Merged Robbert Krebbers requested to merge robbert/vec_countable into master

This is an alternative to !114 (closed), which does not involve defining a partial function from lists to vectors, but makes use of the already existing function list_to_vec.

Also I renamed vec_to_list_of_list into vec_to_list_to_vec. The former appears to be a leftover of the big X_of_Y into Y_to_X rename.

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