Changes since v2.20a (incomplete):

- The Healpix C library has been rewritten almost completely. Algorithms are now
  as similar as possible to the ones in the C++ library, simplifying code
  maintenance. Many functions have become much faster and shorter.
  The interface is unaffected by this change, so existing codes using the
  Healpix C library should not need to be changed.
- An additional set of functions was added to support Nside values up to 2^29.
- Nside values which are not powers of 2 are supported in RING scheme.
- optional compilation without libcfitsio (the cfitsio-related functions are
  skipped in this mode); contributed by Yu Feng