The `prompt` argument can now be either a `str` or `list[str]`.

The change to `generate()` is backwards-compatible.

The changes to `generate_step()`, `top_p_sampling()`, and
`min_p_sampling()` are backwards-incompatible in order to unify shapes;
this could be changed by adding a few if-statements, if preferred.

…server

This basic version only supports bs=1, temp=0, max_kv_size=None.
Supporting samplers, rotating cache, and batching are deferred to future
commits in order to keep this diff small.