Multicomp: SD card controller emulation

The Multicomp machine emulation now includes a SD card interface, working on SD card images. For now hardcoded to a file named "sd.img". Sooner or later a UI feature would be nice to be able to select another image file.

Unrelated to these changes: Much finer synchronization on TraceLog, as profiling revealed that much time was spent by the execution thread on appending to the log.