Deprecate crypto_key_exchange

Deprecate crypto_key_exchange

See #230.
In summary:
The way crypto_key_exchange() works was not optimal.
Several combinations of (private, public) key pairs produce the same
shared secret (see 54a7ef551078dda001f94f1045e142af0fc1008a);
crypto_key_exchange() did nothing to mitigate that.

Since this is a high-level function that cannot be saved even with
documentation because it is so fundamentally misguided,
it is now deprecated (see #232).
Users are encouraged to either adopt the function into their own code
if and only if they require it for backwards compatibility,
starting with the release that removes it,
or preferably migrate to implementing an existing protocol,
such as instantiating a Noise protocol pattern or a protocol like X3DH.