Split up some of the larger source files.

Some of our source files, e.g. engine.c or space.h are getting a bit out of control. Long files are not bad per se, but they make recompiling take longer, and source management a bit more painful (more conflicting edits).

For the sake of sanity, we should split up larger files into groups of functions that operate together, e.g. engine_redistribute.c should contain the engine_redistribute function along with all the mappers and code that is called only from that function itself.

Assigning to myself, but I am under no illusions that I will be able to tackle this any time soon.