add binary exponent return

int bignum_r_degree(struct bn* n)
{
int i = BN_ARRAY_SIZE;
while ((n->array[--i] == 0) && i);
int out = i * WORD_SIZE * 8;
out += 32 - __builtin_clz(n->array[i]);
return out;
}
Returns the number of bits used to write the number (!!!not the buffer size).

Start by locating the bignum_r_degree entry point and reviewing how the bignum array stores words. Implement the requested bit-counting behavior where the issue's snippet belongs, then verify that the result reports the number of significant bits rather than the buffer size using the project's existing checks.

Written by the indexing model from the issue text.