Make FPC use Delphi-compatible generics

[modersohn]

Back when FPC support was added, there was no way to make the Delphi code using generics work with FPC, so a lot of code had to be ifdefed and done differently for FPC. As seen in rkubilius' fork this can now be achieved, so this should be done to reduce the amount of code to maintain.

[modersohn]

This also includes typecasting of nil, as seen in rkubilius@accadfe#diff-36b2c7dd5d553b6fbbdb2e8a0b000dc197f78582939bae38af12a9ac952e95b7R358