Arkworks Elliptic Curve utils overhaul (#1870)

- Removal of Arkworks unit tests. These tests were just testing the
arkworks upstream implementation which should be assumed correct. This
is not the place to test well known dependencies.
- Removal of some over-engineering. We just store the calls to Arkworks
in one file. Per-curve sources are not required.
- Docs formatting

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I also took the opportunity to bump the `bandersnatch-vrfs` crate
revision internally providing some new shiny stuff.