Clean up backend features and vendor curve25519_dalek_derive

[pinkforest]

Closes #414 (again) Implements:

• 529 consensus to use cfg(curve25519_dalek_backend = "override")

• Cleans up the unused backend features which we don't want to advertise for now
• Vendors in unsafe_target_feature in as curve25519-dalek-derive (co-authored by for @koute)
• cfg(curve25519_dalek_diagnostics="build") now emits compile_error build diagnostics used for build script testing
• Found a missing zeroize feature gate -fixed
• build script is now tested with tests/build_tests.sh and added to CI
• Clean up & simplify the documentation for backend override

Notes

• github runner had some weird issue with incremental compilation - fixed
• Closes #532 - see: 1ac7b2 - chose compile_error to simplify gating

[rozbb]

Manually verified that curve25519_dalek_derive/ mirrors the unsafe_target_feature repo at 389ae00, modulo renaming and metadata changes.

[rozbb]

Thanks for the updates! Made a pass on the README which hopefully clarifies how backend selection now works. @koute @tarcieri take a look at just the "Backend" section if you get a chance and lmk if it's good to go.

[koute]

LGTM!

@pinkforest Thank you for pushing this forward!