Add edge case handling for batch_add

[davidnevadoc]

This PR adds a new version of batch_add that accounts for edge cases. (Closes #153)
The old function is now batch_add_nonexceptional ( as suggested #130 (comment))
The new version is batch_add_exceptional

The decrease in performance after the change is minimal:

msm/multicore/10        time:   [5.0757 ms 5.0857 ms 5.0968 ms]
                        change: [+0.9692% +1.2777% +1.5974%] (p = 0.00 < 0.05)
                        Change within noise threshold.
msm/multicore/12        time:   [14.928 ms 14.983 ms 15.013 ms]
                        change: [-0.4586% -0.0956% +0.2811%] (p = 0.63 > 0.05)
                        No change in performance detected.
msm/multicore/14        time:   [23.467 ms 23.691 ms 23.821 ms]
                        change: [+0.2488% +1.2795% +2.4484%] (p = 0.04 < 0.05)
                        Change within noise threshold.
msm/multicore/16        time:   [72.387 ms 73.117 ms 74.886 ms]
                        change: [+0.8051% +2.4199% +5.0474%] (p = 0.04 < 0.05)
                        Change within noise threshold.

The msm functions have been renamed:
multiexp_serial -> serial_multiexp
best_multiexp -> parallel_multiexp
best_multiexp_indepenedent_points -> best_multiexp

I still think this names are quite confusing, so suggestions are welcome. ( Maybe cyclone_ instead of best_ and msm instead of multiexp).

The old batch_add has been kept but is now unsused. We can either remove it, or add another msm or argument to best_multiexp that enables its use.

[davidnevadoc]

Wait for #168 to be merged.

[guorong009]

Overall, I think the PR looks good.

I have 2 comments.

• Is there any use case of batch_add_nonexceptional?
  I think we can get rid of this function if not used.
  It will reduce the code.
  Also, we can keep the original name - batch_add, just add the missing logic.
• How about multiexp_serial/multiexp_parallel, instead of serial_multiexp/parallel_multiexp?
  I just believe that multiexp_* is more meaningful than *_multiexp.

[guorong009]

Regarding the naming, I think we should use msm, instead of multiexp.
Since we are handling the EC points and scalars.

[davidnevadoc]

I'm still on the fence about the naming.

How about multiexp_serial/multiexp_parallel, instead of serial_multiexp/parallel_multiexp?
I don't care about the order really, but best_multiexp breaks the pattern then...

We could have msm_parallel, msm_serial and msm_best. WDYT? @guorong009

[guorong009]

I'm still on the fence about the naming.

How about multiexp_serial/multiexp_parallel, instead of serial_multiexp/parallel_multiexp?
I don't care about the order really, but best_multiexp breaks the pattern then...

We could have msm_parallel, msm_serial and msm_best. WDYT? @guorong009

Yes, I believe they are better.

[kilic]

lgtm :)