This will make it easier to introduce locking

dirvec_find() does not modify the object, thus it should get a const
pointer.

No idea why it was created in directory.h, but it should be in
dirvec.h.

The struct can be forward-declared by other headers, which relaxes the
header dependencies.

Having all functions as static (non-inline) functions generates GCC
warnings, and duplicates binary code across several object files.
Most of dirvec's methods are too complex for becoming inline
functions.  Move them all to dirvec.c and publish the prototypes in
dirvec.h.

Small memory reduction compared to songvec since most users have
much fewer dirs than songs, but still nice to have.