Split calc.c into separate components (dir_(mem|scan|common).c)

The architecture is explained in dir.h. The reasons for these changes is
two-fold:
- calc.c was too complex, it simply did too many things. 399ccdeb is a
  nice example of that: Should have been an easy fix, but it introduced
  a segfault (fixed in 0b49021a), and added a small memory leak.
- This architecture features a pluggable input/output system, which
  should make a file export/import feature relatively simple.

The current commit does not feature any user interface, so there's no
feedback yet when scanning a directory. I'll get to that in a bit.

I've also not tested the new scanning code very well yet, so I might
have introduced some bugs.