FixedBase refactor: rename msm -> mul, declutter the interface and hide aux methods

[mmagician]

Description

I don't think it's worth doing a bigger refactor proposed in #432 - yet still the two MSMs we had were confusing people. This PR is sort of bridging this gap of having a documented, simple interface that is clearly distinct from VariableBaseMSM.

I renamed FixedBase::msm into FixedBase::mul and introduces the following refactors:

• Make all methods except for mul private. (Edit: get_window_table also remains public)
• Simplify the mul interface: takes g and list of scalars as the parameters. The rest of the parameters can be inferred, and the internal methods called from mul directly.
• Add a description and add a doctest

@msmirnov18

closes: #432

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[mmagician]

Actually, I think get_window_table should remain public e.g. for Groth16.

[Pratyush]

What do you think about just moving this to the ScalarMul trait? I think since we're doing a breaking change, a larger refactor would be nice too. I can handle it if you'd like.

[mmagician]

Sure, we can move it. I'm open to other refactors too, the question is how useful this method is.

[mmagician]

What other refactors are you thinking of, actually, aside from moving?

[Pratyush]

Sure, we can move it. I'm open to other refactors too, the question is how useful this method is.

In general this method is useful whenever you want to multiply the same element by different scalars. This is useful primarily in setup methods for, e.g., Groth16 or KZG, but only in testing.

So I'd like to keep it, and just make it easier to use.

What other refactors are you thinking of, actually, aside from moving?

I'll take a stab at these changes, and then we can compare and see if they're worth it.

[Pratyush]

@mmagician I've added my changes; let me know if there's anything further you'd like to change.

[mmagician]

Looks good, I'd approve if I weren't the PR author 👍🏼 Thanks for the changes!

[Pratyush]

Thanks!