faster is_in_correct_subgroup_assuming_on_curve when cofactor is one

[tess-eract]

Description

This is a two-line fix to speed up the function is_in_correct_subgroup_assuming_on_curve for short Weierstrass curve points:
if the cofactor is one, it skips the expensive operation of performing a scalar multiplication by the scalar field's characteristic.

The cofactor is always one when the curve forms part of a cycle, so this is relevant for the curves used for most
recursive SNARKs (e.g. bn254, etc.).

This speeds up operations like CanonicalDeserialize::deserialize_compressed, which can otherwise take a very
long time for large vectors of curve points. It should also reduce the number of constraints needed to allocate
a curve variable.

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• Wrote unit tests
  • existing test coverage should suffice, since no new functionality is introduced.
• Updated relevant documentation in the code
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
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[Pratyush]

Looks good! Thanks for the PR!