Reimplement BN_num_bits_word().

Reimplement BN_num_bits_word().

Provide a simpler and more readable bn_word_clz() function that returns the
number of leading zeros for a given BN_ULONG, then implement
BN_num_bits_word() using bn_word_clz(). This is a hot path and
bn_word_clz() can now be replaced with architecture specific versions where
possible.

ok tb@