C++ wrapper for executable

[sebffischer]

We should create a C++ wrapper object for the Executable with a destructor that clears it after it goes out of scope, otherwise we'll leak memory if an exception is raised during execution.

[sebffischer]

We need to wrap PJRT_LoadedExecutable_GetExecutable_Args in a C++ smart pointer with a destructor.

We should probably do this for all objects.