Lua: vision checking and editing
Sub-task of #42501 but has a separate meaning, in commercial games we can get territory map from a hut (see http://forum.freeciv.org/f/viewtopic.php?f=13&t=91188).
Common API: int (Unit_Type).vision_radius_sq
Client API: bool (Tile).is_seen (known can be checked by .terrain ~= nil but may et a special method)
I've split #42973 ( Tile:show() & Tile:hide() ) and #42974 (default.lua hut code to reveal map) from this.