This has the effect of making `secp256k1_scalar_mul_shift_var` constant
time in both input scalars. Keep the _var name because it is NOT constant
time in the shift amount.
As used in `secp256k1_scalar_split_lambda_var`, the shift is always
the constant 272, so this function becomes constant time, and it
loses the `_var` suffix.
* Make secp256k1_gej_add_var and secp256k1_gej_double return the
Z ratio to go from a.z to r.z.
* Use these Z ratios to speed up batch point conversion to affine
coordinates, and to speed up batch conversion of points to a
common Z coordinate.
* Add a point addition function that takes a point with a known
Z inverse.
* Due to secp256k1's endomorphism, all additions in the EC
multiplication code can work on affine coordinate (with an
implicit common Z coordinate), correcting the Z coordinate of
the result afterwards.
Refactoring by Pieter Wuille:
* Move more global-z logic into the group code.
* Separate code for computing the odd multiples from the code to bring it
to either storage or globalz format.
* Rename functions.
* Make all addition operations return Z ratios, and test them.
* Make the zr table format compatible with future batch chaining
(the first entry in zr becomes the ratio between the input and the
first output).
Original idea and code by Peter Dettman.
