precomp: New directory for precomputed files.

The `precomp' directory in the distribution tarball contains pre-built
tables, for CRC and unihash computations.  Distributing the tables means
that the library can be built by cross-compilers.  Indeed, we don't even
try to generate the tables if cross compilation is detected.

The precomp directory shouldn't exist in revision-control, but it's
populated during the build process if necessary, and it's distributed in
release tarballs.