整理代码，新增string扩展模块，把一些小模块合并进string和table扩展模块中

parts/modes/pctrain_n.lua

@@ -59,7 +59,7 @@ return{
 		mText(drawableText.pc,69,412)
 	end,
 	score=function(P)return{P.stat.pc,P.stat.time}end,
-	scoreDisp=function(D)return D[1].." PCs   "..TIMESTR(D[2])end,
+	scoreDisp=function(D)return D[1].." PCs   "..STRING.time(D[2])end,
 	comp=function(a,b)return a[1]>b[1]or a[1]==b[1]and a[2]<b[2]end,
 	getRank=function(P)
 		local L=P.stat.pc