Mistake with mpn_sqr_n

[Torbjorn Granlund]

In GMP 5.0, we introduced mpn_sqr as a documented function.
There had been an internal function mpn_sqr_n before.

Now, after a bug report (1) I realise that mpn_sqr_n is documented in
GMP 4.3.2 (but not in GMP 4.3.1).  That was never our intention; we
don't want mpn_sqr_n public, and a patch release should certainly not
change the API.

I am not sure how to handle this situation.  We could perhaps

1. Treat it like a documentation error and tell people that the function
   is not supported and indeed gone from GMP 5.  We could then patch GMP
   4.3 and release GMP 4.3.3 with the documentation error fixed.

2. Make future GMP releases compatible with this documentation error,
   which means that we need to re-introduce mpn_sqr_n (which would just
   jump to mpn_sqr).  GMP 5.0.0 and GMP 5.0.1 will be incompatible, and
   we would against our practices have to introduce another API change
   with GP 5.0.2.

Thoughts?

(1) http://gmplib.org/list-archives/gmp-discuss/2010-November/004365.html

-- 
Torbjörn