Windows下Unicode(UCS2)，UTF8，GBK(GB2312)互转

Windows下提供了两个编码转换函数：WideCharToMultiByte和MultiByteToWideChar。

用它们可实现Unicode（UCS2），UTF8，GBK（GB2312）互转。这两个函数的原型是：

int
WINAPI
MultiByteToWideChar(
__in UINT     CodePage,
__in DWORD    dwFlags,
__in_bcount(cbMultiByte) LPCSTR   lpMultiByteStr,
__in int      cbMultiByte,
__out_ecount_opt(cchWideChar) __transfer(lpMultiByteStr) LPWSTR  lpWideCharStr,
__in int      cchWideChar);

int
WINAPI
WideCharToMultiByte(
__in UINT     CodePage,
__in DWORD    dwFlags,
__in_ecount(cchWideChar) LPCWSTR  lpWideCharStr,
__in int      cchWideChar,
__out_bcount_opt(cbMultiByte) __transfer(lpWideCharStr) LPSTR   lpMultiByteStr,
__in int      cbMultiByte,
__in_opt LPCSTR   lpDefaultChar,
__out_opt LPBOOL  lpUsedDefaultChar);