Update JSON.ahk

Use Lookup-table is a little more effective.
Test:
```ahk
t:=["0","1","2","3","4","5","6","7","8","9","A","B","C","D","E","F"]
b:=A_TickCount
Loop 1000000{
	y:=""
	Loop 0x10
		;~ y.= Chr((x:=A_Index-1) + (x<10 ? 48 : 55))	;23500 ms
		y.=t[A_Index]	;13063 ms
}
MsgBox % A_TickCount - b
ExitApp
```