Expose the ed25519 API

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This adds a low-level APIs to perform computations over the edwards25519 curve, only useful to implement custom constructions.

Exposed functions: crypto_ed25519_scalarmult/2, crypto_ed25519_scalarmult_base/1, crypto_ed25519_scalarmult_noclamp/2, crypto_ed25519_scalarmult_base_noclamp/1, crypto_ed25519_add/2, crypto_ed25519_sub/2, crypto_ed25519_is_valid_point/1, crypto_ed25519_scalar_reduce/1, crypto_ed25519_scalar_negate/1, crypto_ed25519_scalar_add/2, crypto_ed25519_scalar_sub/2, crypto_ed25519_scalar_mul/2