Replace partitionScalarOld by partitionScalar in batchScalarMul

[gbotrel]

• checkpoint
• feat: replace partitionScalarOld by partitionScalar

[yelhousni]

LGTM 👍
We might revisit partitionScalars for secp256k1 to allow a path when a spare bit is not available.

[gbotrel]

ha, well, I think this new partitionScalars would work as is; the returned values is a list of []uint16 digits, so normally we would just have a last window that's larger than c. The only issue is that... if we have c = 16 and lastC = 17, the digits don't fit on uint16 🙃
(so... as a cheat, if in the template for secp we have a special case to limit the choice of c to 15, we should be good.)