Implement graceful exit, fix memory leaks & incorrect memory deallocation

[and3rson]

This PR introduces several improvements:

• exit()/atexit() is replaced with setjmp()/longjmp() combo to allow graceful shutdown with cleanup handler. In a nutshell, exit() is replaced with main_exit() which performs longjmp. So now Nofrendo can be exited with event_quit.
• In nes/nes_rom.c, memory for rom and vrom was allocated WITHOUT using guards (mem_alloc) but freed WITH using guards (NOFRENDO_FREE). This caused the "mem_deleteblock ... -- block not found" which called exit() and thus crashed the app. This is now fixed.
• Memory corruption in bitmap.c is still present, however it doesn't seem to cause any issues. So I replaced ASSERT_MSG with a log message to treat corrupted memory blocks as non-critical issue and proceed with a normal shutdown.

All changes tested on actual hardware: I was able to successfully exit the emulator back into my main firmware with no memory leaks.

[moononournation]

This is a Arduino library repository. If you change the API, please also update the example.

[moononournation]

why sometimes change NOFRENDO_FREE() to free()?