Features
    * Add Ed25519 curve.
    * Add mbedtls_eddsa_sign() for EdDSA signatures. It supports pure, ctx
      and prehash signature types, for Ed25519 curve.
    * Add mbedtls_eddsa_verify() for EdDSA signatures verification.
    * Add mbedtls_eddsa_read_signature() and mbedtls_eddsa_write_signature()
      for reading from ASN.1 and writing to ASN.1 signatures.
    * Add Ed448 curve.
    * Add EdDSA signatures for Ed448 curve. It supports pure and prehash.
