Removed undesirable typecast to WideString

Runtime: PSGetUnicodeString was same as PSGetWideString before. I removed undesirabled typecasts to WideString and let to do compiler default conversions instead. Runtime is faster now.

Compiler: GetUnicodeString was same as GetWideString before. I removed undesirabled typecasts to WideString and let to do compiler default conversions instead.

String 'plus' operations was compiled with WideString result. Changed to UnicodeString, because it is much faster on Unicode Delphi. On non-unicode systems is btUnicodeString mapped to btWideString, as before.

All this made PascalScript much faster on Delphi 2009+.