Rewrite and simplify bn_sqr()/bn_sqr_normal().

Rewrite and simplify bn_sqr()/bn_sqr_normal().

Rework bn_sqr()/bn_sqr_normal() so that it is less convoluted and more
readable. Instead of recomputing values that the caller has already
computed, pass it as an argument. Avoid branching and remove duplication
of variables. Consistently use a_len and r_len naming for lengths.

ok tb@