feat: new fs.calc_size() API

[sxyazi]

Follow up to #2683

How to use:

local total = 0
local it = fs.calc_size(Url("/tmp/yazi"))

while true do
  local found = it:recv()
  if found then
    ya.dbg("found: " .. found)
    total = total + found
  else
    break
  end
end

ya.dbg("done! total: " .. total)

Output:

found: 1956532522
found: 911553073
found: 1100032563
found: 1079350830
found: 943483457
found: 1160212602
found: 490316018
found: 821866
found: 104071422
found: 256609193
found: 512100934
found: 473746322
found: 957889320
found: 1439720840
found: 59380140
done! total: 11445821102