BSTR support in the P/Invoke tool

Strings used by COM methods are passed as BSTRs. My P/Invoke command-line tool previously had no way to use such methods, except possibly by manually constructing the BSTR structure, which would have been a huge pain. So today, I introduced a slot kind that manages text as BSTRs. Implementation was made very easy by .NET's PtrToStringBSTR and StringToBSTR methods.