Floating-point types in the P/Invoke tool

Even though I'm not aware of any standard Win32 API functions that take floating-point values, it would be good to support such data in my P/Invoke command-line tool. That turned out to be pretty simple: just adding single and double kinds mapped to the appropriate .NET types did the job. The .NET marshal already knew how to transfer those types, so such values can be serialized and recovered successfully.