Use utf8.char, tup supports lua 5.3 after all

Did it gain support in a later version? Or, was I imagining it only
supporting 5.1 from the start?

util/lua-ext.lua

@@ -19,15 +19,3 @@ function astable(table)
         return {}
     end
 end
-
--- Lua 5.1, which ships with Tup, does not have a way of creating a string from
--- a unicode codepoint...
--- 
--- https://stackoverflow.com/a/7799843
-function unichr(ord)
-    if ord == nil then return nil end
-    if ord < 32 then return string.format('\\x%02x', ord) end
-    if ord < 126 then return string.char(ord) end
-    if ord < 65539 then return string.format("\\u%04x", ord) end
-    if ord < 1114111 then return string.format("\\u%08x", ord) end
-end

util/pokemon.lua

@@ -2,7 +2,7 @@
 function decodeFS(filename)
     return filename:gsub('_(%x%x%x%x)',
                    function(hex)
-                       return unichr(tonumber(hex, 16))
+                       return utf8.char(tonumber(hex, 16))
                    end)
 end