Make udprelay/crypto_io public

[XOR-op]

Since the ProxySocket interface is not versatlie enough (i.e. require a ShadowUdpSocket under it), developers may need write their own structs based on cryptography tools the crate offers. However, the corresponding module udprelay/crypto_io is private, making this difficult.

This pull request makes udprelay/crypto_io module public to solve this problem. It's also the counterpart of tcprelay/crypto_io, which has been public in the latest release.

[zonyitoo]

How developers use the functions in crypto_io?

[XOR-op]

For example, use encrypt_client_payload to encrypt udp packets.

[zonyitoo]

So the connection is not UDP?

[XOR-op]

I don't quite get your meaning. But developers may want other underlying "connections", such as TCP, another Shadowsocks UDP stream or some homebrew protocols. Since ProxySocket does not support this, the easiest way is to export the underlying cryptography methods.