Allow train priority to be a signed int

[hurricup]

I don't really know the reason behind the train priority being only a byte, probably some optimization, but it is feels like unnecessary headache.

When I want to automate the prioritization, i have to normalize it every time. For example: I have several stations, and I want to balance them automatically:
- I compute item demand, divide it by the cart capacity and getting the train limit.
- but, if I want stations to be fed evenly, I'd like to use a `-items_count` as priority. But I can't do this. Priority is set, but not working. And wiki says that priority should be a number from the byte range. So I have to normalize it and this feels like unnecessary boilerplate, taking space and unnecessarily complicating the circuit.