RFC3629 - UTF-8, a transformation format of ISO 10646

convert_UTF16_to_UTF8

// RFC3629 - UTF-8, a transformation format of ISO 10646 -
// +------+-----------+-------------------------------------+-----------+-----------+-----------+-----------+
// | Rule | UTF-16    | Representation                      | UTF-8 1st | UTF-8 2nd | UTF-8 3rd | UTF-8 4th |
// +------+-----------+-------------------------------------+-----------+-----------+-----------+-----------+
// | [1]  |           |                   00000000 0zzzzzzz | 0zzz zzzz |           |           |           |
// |      |           +-------------------------------------+           |           |           |           |
// |      | 0x000000~ |                   00000000 00000000 | 0000 0000 |           |           |           | (0x00)~
// |      | 0x00007F  |                   00000000 01111111 | 0111 1111 |           |           |           | (0x7F)
// +------+-----------+-------------------------------------+-----------+-----------+-----------+-----------+
// | [2]  |           |                   00000yyy yyzzzzzz | 110y yyyy | 10zz zzzz |           |           |
// |      |           +-------------------------------------+           |           |           |           |
// |      | 0x000080~ |                   00000000 10000000 | 1100 0010 | 1000 0000 |           |           | (0xC2,0x80)~
// |      | 0x0007FF  |                   00000111 11111111 | 1101 1111 | 1011 1111 |           |           | (0xDF,0xBF)
// +------+-----------+-------------------------------------+-----------+-----------+-----------+-----------+
// | [3]  |           |                   xxxxyyyy yyzzzzzz | 1110 xxxx | 10yy yyyy | 10zz zzzz |           |
// |      |           +-------------------------------------+           |           |           |           |
// |      | 0x000800~ |                   00001000 00000000 | 1110 0000 | 1010 0000 | 1000 0000 |           | (0xE0,0xA0,0x80)~
// |      | 0x000FFF  |                   00001111 11111111 | 1110 0000 | 1011 1111 | 1011 1111 |           | (0xE0,0xBF,0xBF)
// |      |           |                                     |           |           |           |           |
// |      | 0x001000~ |                   00010000 00000000~| 1110 0001 | 1000 0000 | 1000 0000 |           | (0xE1,0x80,0x80)~
// |      | 0x00C7FF  |                   10000111 11111111 | 1110 1100 | 1011 1111 | 1011 1111 |           | (0xEC,0xBF,0xBF)
// |      |           |                                     |           |           |           |           |
// |      | 0x00E000~ |                   11100000 00000000~| 1110 1110 | 1000 0000 | 1000 0000 |           | (0xEE,0x80,0x80)~
// |      | 0x00FFFF  |                   11111111 11111111 | 1110 1111 | 1011 1111 | 1011 1111 |           | (0xEF,0xBF,0xBF)
// |      | 0x00FEFF  | (BOM)             11111110 11111111 | 1110 1111 | 1011 1011 | 1011 1111 |           | (0xEF,0xBB,0xBF)
// +------+-----------+-------------------------------------+-----------+-----------+-----------+-----------+
// | [4]  |           |          000wwwxx xxxxyyyy yyzzzzzz | 1111 0www | 10xx xxxx | 10yy yyyy | 10zz zzzz |
// |      |           +-------------------------------------+           |           |           |           |
// |      | 0x010000~ |          00000001 00000000 00000000 | 1111 0000 | 1001 0000 | 1000 0000 | 1000 0000 | (0xF0,0x90,0x80,0x80)~
// |      | 0x10FFFF  |          00010000 11111111 11111111 | 1111 0100 | 1000 1111 | 1011 1111 | 1011 1111 | (0xF4,0x8F,0xBF,0xBF)
// +------+-----------+-------------------------------------+-----------+-----------+-----------+-----------+
//
// First UTF-16 data(0xD800 ~ 0xDBFF) and second UTF-16 data(0xDC00 ~ 0xDFFF) is Surrogate Pairs.
//
// +------+-----------+-------------------------------------+-----------+-----------+-----------+-----------+
// | Rule | UTF-16    | Surrogate Pairs to UTF-8            | UTF-8 1st | UTF-8 2nd | UTF-8 3rd | UTF-8 4th |
// +------+-----------+-------------------------------------+-----------+-----------+-----------+-----------+
// | [5]  |           |                                     |           |           |           |           |
// |      |           | 110110UU UUwwwwxx 110111yy yyzzzzzz | 1111 0uuu | 10uu wwww | 10xx yyyy | 10zz zzzz |
// |      |           +-------------------------------------+           |           |           |           |
// |      | D800,DC00 | 11011000 00000000 11011100 00000000 | 1111 0000 | 1001 0000 | 1000 0000 | 1000 0000 | (0xF0,0x90,0x80,0x80)~
// |      |   |    |  |                                     |           |           |           |           |
// |      | DBFF,DFFF | 11011011 11111111 11011111 11111111 | 1111 0100 | 1000 1111 | 1011 1111 | 1011 1111 | (0xF4,0x8F,0xBF,0xBF)
// +------+-----------+-------------------------------------+-----------+-----------+-----------+-----------+
//
//  Surrogate Pairs convert to UTF-8 steps.
//
//      step1. read two UTF-16 data
//      step2. (0xUUUU + 1) to 0xuuuuu
//      step3. Apply Rule[4] to value(0x010000 ~ 0x10FFFF)
//