FEAT: new ICONV native for conversion between various code pages

This function is on Posix wrapped around [iconv](http://man7.org/linux/man-pages/man1/iconv.1.html) function. On Windows it is emulated using Kernel functions for conversion between wide char and multi byte.

So far there are two modes, conversion from binary to unicode string:
```
>> iconv #{9AE96D} 'cp1252
== "šém"
```
or from one binary to another with different encoding:
```
>> iconv/to #{9AE96D} 'cp1252 'utf-8
== #{C5A1C3A96D}
```

Author: Oldes