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

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.

mentioned in merge request !114 (closed)

added 6 commits

• cb08e14f - Put `LeibnizEquiv` lemmas for `dom` in section.
• 78f50186 - Add `SetUnfold` instances for `dom`.
• 122a003c - Rename `vec_to_list_of_list` into `vec_to_list_to_vec`.
• 1fa6b9b0 - Prove lemma `list_to_vec_to_list`.
• 0aa3e165 - Add `Countable` instance for `vec`.
• 58fdf492 - CHANGELOG entry.

Compare with previous version

resolved all threads

@tchajed Any remarks on this MR?

added 9 commits

• 58fdf492...4fc67339 - 5 commits from branch master
• 74e74b5a - Rename `vec_to_list_of_list` into `vec_to_list_to_vec`.
• 85f50214 - Prove lemma `list_to_vec_to_list`.
• 8b9dd4e5 - Add `Countable` instance for `vec`.
• 2d2a72ae - CHANGELOG entry.

Compare with previous version

mentioned in issue #45 (closed)

LGTM, feel free to merge and close !114 (closed).

merged

mentioned in commit 7d705c84