[FEA]: Enable assertions by default in CMake Debug builds

[bernhardmgruber]

I got asked by someone what the state of the art way to enable assertions, e.g. to trigger on OOB access on a cuda::std::span is for CCCL. I replied that we cannot do anything for plain nvcc (yet), because we don't know whether the users intends to have a debug or release build. But if CCCL is consumed via CMake, we would look at the CMAKE_BUILD_TYPE and define CCCL_ENABLE_ASSERTIONS. Or are we?

I did not find where CMake would define CCCL_ENABLE_ASSERTIONS when any of the CCCL libraries is imported, so I conclude that we don't enable assertions anywhere. I think we should enable them for CMake Debug builds.

[jrhemstad]

I think we should enable them for CMake Debug builds.

Agreed.

[bernhardmgruber]

@Kh4ster would love to have this soon