SSE UTF16 => latin1

[Nick-Nuon]

EDIT: Benchmarks for lastest commits:

convert_utf16_to_latin1+haswell, input size: 864610, iterations: 30000, dataset: /home/leorio/unicode_lipsum/wikipedia_mars/french.utflatin16.txt
   0.345 ins/byte,    0.407 cycle/byte,    7.853 GB/s (1.3 %),     3.198 GHz,    0.848 ins/cycle 
   0.345 ins/char,    0.407 cycle/char,    7.853 Gc/s (1.3 %)     1.00 byte/char 
convert_utf16_to_latin1+icelake, input size: 864610, iterations: 30000, dataset: /home/leorio/unicode_lipsum/wikipedia_mars/french.utflatin16.txt
   0.143 ins/byte,    0.448 cycle/byte,    6.909 GB/s (1.2 %),     3.097 GHz,    0.318 ins/cycle 
   0.143 ins/char,    0.448 cycle/char,    6.909 Gc/s (1.2 %)     1.00 byte/char 
convert_utf16_to_latin1+iconv, input size: 864610, iterations: 30000, dataset: /home/leorio/unicode_lipsum/wikipedia_mars/french.utflatin16.txt
  17.011 ins/byte,    3.024 cycle/byte,    1.056 GB/s (29.1 %),     3.193 GHz,    5.625 ins/cycle 
  34.022 ins/char,    6.048 cycle/char,    0.528 Gc/s (29.1 %)     2.00 byte/char 
WARNING: Measurements are noisy, try increasing iteration count (-I).
convert_utf16_to_latin1+icu, input size: 864610, iterations: 30000, dataset: /home/leorio/unicode_lipsum/wikipedia_mars/french.utflatin16.txt
   3.444 ins/byte,    0.763 cycle/byte,    4.187 GB/s (1.1 %),     3.195 GHz,    4.513 ins/cycle 
   3.444 ins/char,    0.763 cycle/char,    4.187 Gc/s (1.1 %)     1.00 byte/char 
convert_utf16_to_latin1+westmere, input size: 864610, iterations: 30000, dataset: /home/leorio/unicode_lipsum/wikipedia_mars/french.utflatin16.txt
   1.251 ins/byte,    0.269 cycle/byte,   11.889 GB/s (1.3 %),     3.199 GHz,    4.649 ins/cycle 
   1.251 ins/char,    0.269 cycle/char,   11.889 Gc/s (1.3 %)     1.00 byte/char 


convert_utf16_to_latin1_with_errors+haswell, input size: 864610, iterations: 30000, dataset: /home/leorio/unicode_lipsum/wikipedia_mars/french.utflatin16.txt
   3.251 ins/byte,    0.743 cycle/byte,    4.300 GB/s (1.2 %),     3.195 GHz,    4.375 ins/cycle 
   3.251 ins/char,    0.743 cycle/char,    4.300 Gc/s (1.2 %)     1.00 byte/char 
convert_utf16_to_latin1_with_errors+icelake, input size: 864610, iterations: 30000, dataset: /home/leorio/unicode_lipsum/wikipedia_mars/french.utflatin16.txt
   3.251 ins/byte,    0.738 cycle/byte,    4.326 GB/s (1.1 %),     3.195 GHz,    4.402 ins/cycle 
   3.251 ins/char,    0.738 cycle/char,    4.326 Gc/s (1.1 %)     1.00 byte/char 
convert_utf16_to_latin1_with_errors+westmere, input size: 864610, iterations: 30000, dataset: /home/leorio/unicode_lipsum/wikipedia_mars/french.utflatin16.txt
   5.376 ins/byte,    0.907 cycle/byte,    3.523 GB/s (1.8 %),     3.194 GHz,    5.928 ins/cycle 
   5.376 ins/char,    0.907 cycle/char,    3.523 Gc/s (1.8 %)     1.00 byte/char  ```

[lemire]

Looks good.

[Nick-Nuon]

I think this one is good to go

[lemire]

Merging.