Fix performance regression

[snowleopard]

Zero build jumped from 6 seconds to 65 on my machine after I fixed these two issues: #15 and #19. Not sure what the reason is yet, but I suspect findModuleFiles in Settings.hs.

[snowleopard]

The issue is actually in Rules/Generate.hs which runs expensive findModuleFiles too often.

[snowleopard]

To improve performance I implemented caching for findModuleFiles, see Oracles/ModuleFiles.hs in cf825fe. Zero build time is back to 6 sec.

Further performance improvements are possible, I'll keep working on this.

[snowleopard]

Further work on this issue: 013fa90. Oracles/ModuleFiles.hs got a lot simpler, and average zero build time is down to 5 sec. There is further room for improvement (some parts of the source tree are scanned twice).