Remove UnsafeCell (Attempt 2)

[davxy]

Superseeds #87

• UnsafeCell removal
• Expose parallel feature

[davxy]

I'm not sure if we want to make this public so that the user can eventually keep it and manually invoke resplit if required.

If you want to make it private then some methods can be removed as well... as are not used

[davxy]

@burdges do you think this is good to be merged?

[burdges]

I see, 2af5bd2 reverts 4c7eba5. I'll go through the finished change, not the individual commits. ;)

[burdges]

Is this working purely because <K: AffineRepr> exists in the struct? Is this a new rust feature?

[davxy]

K is bound only by the AffineRepr trait. There are no ambiguities for the ScalarField associated type

[davxy]

I see, 2af5bd2 reverts 4c7eba5. I'll go through the finished change, not the individual commits. ;)

Yeah. I decided to revert because that modification was using the secret for the computation: res = (A+B)*c = secret*c).
The original code was better as it doesn't reconstruct the secret to compute the result res = (A*c + B*c).

Doing the computation with the reconstructed secret makes the splitting trick pointless

[burdges]

Appears I never fixed this. I'd basically come around to a reasonable compromize, but I never made the changes?!? fuck