Avoid division when decomposing scalars

- In secp256k1_gej_split_exp, there are two divisions used. Since the denominator is a constant known at compile-time, each can be replaced by a multiplication followed by a right-shift (and rounding).
- This change adds the constants g1, g2 for this purpose and rewrites secp256k1_gej_split_exp accordingly.
Remove secp256k1_num_div since no longer used