Add methods to convert from ed25519 to curve25519

[sekhat]

Adds to_curve25519 to both ed25519 SecretKey and PublicKey returning curve25519 SecretKey and PublicKey respectively.

[sekhat]

Seems the builds are failing due to something outside my code:

error: the item `hex` is imported redundantly

   --> src/crypto/hash/sha256.rs:67:13

    |

67  |         use hex;

    |             ^^^

    | 

   ::: src/lib.rs:115:1

    |

115 | extern crate hex;

    | ----------------- the item `hex` is already imported here

    |

note: lint level defined here

   --> src/lib.rs:88:5

    |

88  |     unused,

    |     ^^^^^^

    = note: #[deny(unused_imports)] implied by #[deny(unused)]

error: the item `hex` is imported redundantly

   --> src/crypto/hash/sha512.rs:55:13

    |

55  |         use hex;

    |             ^^^

    | 

   ::: src/lib.rs:115:1

    |

115 | extern crate hex;

    | ----------------- the item `hex` is already imported here

error: aborting due to 2 previous errors

error: Could not compile `rust_sodium`.

[sekhat]

Submitted pull request #110 to fix the build issue.

[S-Coyle]

The rust_sodium crate is no longer maintained by MaidSafe, you should check whether it is actively maintained on Subspace as per the readme, or switch to a modern alternative.