Add ec:msm:{Fixed,Variable}Base:msm_checked_len

[vlopes11]

Description

Add the new methods for checked fixed and variable base multi-scalar
multiplication. They will return None if the arguments don't have the
same length.

This way, the users may confidently call this function without having to
check the length themselves.

Also, ec:msm:{Fixed,Variable}BaseMSM was renamed to
ec:msm::{Fixed,Variable}Base to avoid redundancy. The import path
contained msm several times, and now it contains only one incidence
for the type resolution and one for the function call.

Closes #319

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• Wrote unit tests
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