Negative line indices in `set_lines`

[GordianDziwis]

From the nvim_buf_set_lines docs:

    Indexing is zero-based, end-exclusive. Negative indices are interpreted as
    length+1+index: -1 refers to the index past the end. So to change or
    delete the last line use start=-2 and end=-1.

But the range in nvim-oxi is usize, so the negative indexing is not possible.

Is this intended?