debug: add assertions for easier engine debugging

[jorgegv]

Add DEBUG_ASSERT calls in all engine modules to check for conditions that should be met at all times.

DEBUG_ASSERT calls only get compiled when building with RAGE1_DEBUG=1 environment, and are defined to null if that variable is not defined (so no impact in production).

The need for this has been identified by spurious hangs when playing FAMARGON (heavy sprite, memory and tile usage)

Modules to examine:

• main
• memory
• dataset
• codeset
• sp1
• hero
• game_loop
• map
• enemy