Initial scrypt support

Work on scrypt in libsodium has begun:

https://github.com/jedisct1/libsodium/commits/scrypt

This implements it, and tests it against an scrypt test vector. It passes for me! It requires some hacks to libsodium to work though, since the scrypt work there isn't fully complete (the `crypto_scrypt` symbol needs to be exported)

This still needs a way to fail gracefully if the `crypto_scrypt` symbol isn't present in libsodium.

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**Update**: It's good to go on the latest libsodium 0.5.0 release now!