/// <summary>
/// 지정한 char 의 unicode string을 <paramref name="writer"/>에 쓴다.
/// </summary>
/// <param name="writer"></param>
/// <param name="c"></param>
public static void WriteCharAsUnicode(TextWriter writer, char c) {
writer.ShouldNotBeNull("writer");
var h1 = HexTextTool.IntToHex((c >> 12) & '\x000f');
var h2 = HexTextTool.IntToHex((c >> 8) & '\x000f');
var h3 = HexTextTool.IntToHex((c >> 4) & '\x000f');
var h4 = HexTextTool.IntToHex(c & '\x000f');
writer.Write('\\');
writer.Write('u');
writer.Write(h1);
writer.Write(h2);
writer.Write(h3);
writer.Write(h4);
}