Handle zero-case in group scalar multiplication

[slumber]

Description

closes: #94

See doc-comments for more details.

This part of code could as well be moved into to_affine, but this exact behavior might not be expected outside of scalar_mul

---

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[Pratyush]

Thanks for the PR! Could you fix the errors in CI and then I'll merge this.

[slumber]

Thanks for the PR! Could you fix the errors in CI and then I'll merge this.

I don't think these errors are coming from my PR, perhaps from the CI setup. I don't mind fixing it, but then it would bring some unrelated lines here.

error: call to `.borrow()` on a reference in this situation does nothing
   --> src/groups/curves/twisted_edwards/mod.rs:340:44
    |
340 |                 let base_power = base_power.borrow();
    |                                            ^^^^^^^^^ help: remove this redundant call
    |
    = note: the type `ark_ec::twisted_edwards::Projective<P>` does not implement `Borrow`, so calling `borrow` on `&ark_ec::twisted_edwards::Projective<P>` copies the reference, which does not do anything and can be removed

[slumber]

@Pratyush should be fixed now

UPD: or not

[slumber]

Requires https://github.com/arkworks-rs/curves/pull/167

[Pratyush]

@slumber would you like to add a changelog entry for this?

[slumber]

@slumber would you like to add a changelog entry for this?

Yes, will do