Pass pointers to states rather than LIDs

[alessandropellegrini]

Refactoring the code base making all functions get a pointer to the state of a LP rather than its LID would make everything far more readable.

To this end, we should apply a cartesian product to all memory-related buffers, which are still contiguous.

[alessandropellegrini]

The same should be done as well for Thread control blocks, introducing a current_thread TLS variable.

[alessandropellegrini]

When addressing this issue, also serial engine should be refactored to rely on LP_State struct, for uniformity.