Moving BSTR conv to native code in SecureStringToBSTR.

Moving BSTR conv to native code in SecureStringToBSTR.

Previous implementation laid out the BSTR structure
in directly memory in C# code. This layout was not correct
for windows.
Furthermore memory was allocated using AllocCoTaskMem but later freed
using SysFreeString which on windows are different memory areas.
Third issue is that SecureString asumes big-endian when copying the chars
to its internal byte buffer. So on little-endian environments,
a swap of bytes is needed before the buffer is sent to native side
for BSTR conversion. This should probably be handled internally in
SecureString instead.