Freeciv

Make a bunch of universal converters that turn lua_Object into various API classes. This set is to be used in find module utilities and any other API functions, to make polymorphism uniform.

• The only sane converter between API types I can imagine is Nation_Type<->Player. Other, like Unit->Unit_Type, should be left to explicit functions.
• Things that have id's and/or rule_name's are converted from numbers and/or strings in obvious way.
• Things that have non-rule names (players and cities, maybe in the future units) should better be left to explicit find.*() calls, though for players maybe we can use string calls uniformly.
• Tiles should be findable by a table with coordinates, by default {nat_x, nat_y} in natural coordinates (as current find.tile(a, b) does), but should also support named {nat_x=1, nat_y=1} and, with some priority, {x=1,y=1} for map coords and {native_x=1, native_y=1} for native pos.