Replace bn_sqr_words() with bn_sqr_add_words().

Replace bn_sqr_words() with bn_sqr_add_words().

In order to implement efficient squaring, we compute the sum of products
(omitting the squares), double the sum of products and then finally
compute and add in the squares. However, for reasons unknown the final
calculation was implemented as two separate steps.

Replace bn_sqr_words() with bn_sqr_add_words() such that we do the
computation in one step, avoid the need for temporary BN and remove
needless overhead. This gives us a performance gain across most
architectures (even with the loss of sse2 on i386, for example).

ok tb@