CharsetConverter: add independent implementation of W->UTF-8 and UTF-8->W converters

To resolve some circular initialization dependency on Win32 (CLog initialization use CCharsetConverter functions which can call CLog functions), I add implementation of simpleUtf8ToW() and simpleWToUtf8().
Those functions can be used safely during startup and shutdown, from initializers of global or static objects as they use no locking, no external objects.
New specialized functions should be faster than libiconv functions as libiconv call transformation function for each symbol to convert from given encoding to internal encoding and then from internal encoding to target encoding.
Tests for new function are included in this PR.