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.

The technique is discussed, amongst other places, in the paper "Efficient Software Implementation of Public-Key Cryptography on Sensor Networks Using the MSP430X Microcontroller" (Gouvea, Oliveira, Lopez) - in section 4.3.