Simplify buildsystem.
Bind crypto_box_* family of functions.
Bind crypto_hash_* family of functions.
Bind crypto_scalarmult_* family of functions.
Bind crypto_verify_* family of functions (used internally).
Bind crypto_sign_* family of functions.
Bind crypto_secretbox_* family of functions.
Bind crypto_stream_* family of functions.
Bind crypto_auth_* family of functions.
Update README.
Update to use the new zero-copy ctypes interface.
Add *_key aliases for * key types.

When using ocaml-sodium in downstream code, without `open Sodium'
it is very annoying to write `Sodium.public Sodium.Box.key' and so on.
Bigstring → Bigbytes.
String → Bytes.
the bytes package is required for the library and the tests

this retains compatibility with ocaml less than 4.02
ocaml_bytes → ocaml_string in ctypes 0.3.0
80 char line convention
Update README for API changes, new minimum libsodium req; start changelog