Implement lists

- The list class is currently incomplete in that it does not implement
the full “sequence interface”.
- Lists can be created with the following syntax: `[x, y, z, …]`
- Supported methods (so far): `append`, `pop`, `insert`
- List lengths can be obtained via the `len` field; this will likely be
changed in the near future.
- List indices can be read (`list[n]`) and written to (`list[n] = x`).
- Compound list assignment (e.g. `list[n] += x`) is on the to-do list.