Fixes for latest Nim

[RSDuck]

• useWinUnicode was removed a while ago, now typically only the Unicode APIs are used (see remove decades-deprecated Win32 API *A function support nim-lang/Nim#21315).
• Deriving exceptions directly off Exception generated some warnings saying that they should be derived of a more concrete exception type.
• Putting together a set of int consts/literals would previously create an int16 set, now it's an error. So I replaced the set by an array.
• Getting the pointer of the wide string by casting it to pointer seems to not work anymore? It works this way (which I think should also be more stable).

Only tested on Windows, there might still remain issues on other platforms.

[euantorano]

Thanks so much for the contribution.

A quick glance through looks good, but I don't currently have my USB to RS232 adapters to hand to actually test with. I'm going to merge this, but not tag a new release just yet until I've had a chance to test it myself.