Pass Rust closures where the Lua api expects functions

[noib3]

It should be possible to pass a Rust closure everywhere one would pass a Lua function when using the Lua API.

Just as an example, something like

local print_current_modified = function()
  local buf = vim.api.nvim_get_current_buf()

  local is_modified = vim.api.nvim_buf_call(buf, function()
    return vim.api.nvim_buf_get_option(0, "modified")
  end)

  print(is_modified)
end

should become

use nvim_oxi as nvim;

fn print_current_modified() -> nvim::Result<()> {
    let buf = nvim::api::get_current_buf();

    let is_modified = buf.call(|| {
        nvim::Buffer::from(0).get_option::<bool>("modified")
    })?;

    nvim::print!("{is_modified}");

    Ok(())
}

Functions should accept FnOnce or FnMut closures on a case-by-case basis, but always with a 'static bound.