Reimplement bn_sqr_comba{4,8}().

Reimplement bn_sqr_comba{4,8}().

Use bignum primitives rather than the current mess of macros.The sqr_add_c
macro gets replaced with bn_mulw_addtw(), while the sqr_add_c2 macro gets
replaced with bn_mul2_mulw_addtw().

The variables in the comba functions have also been reordered, so that the
patterns are easier to understand - the compiler can take care of
optimising the inputs and outputs to avoid register moves.

ok tb@