Improve secp256k1_point_unsafe using precomputed tables

[bigspider]

#276 introduced a faster version of secp256k1_point that is not secured against side channels, for use where that's not a concern (public data). However, there is still a vast room for improvement, as the current code use a generic elliptic curve scalar multiplication method, while secp256k1_point multiplies a fixed point (the generator) by a scalar.

Precomputed tables of multiples could still improve performance by a large margin, at the cost of an increase in binary size.