Implement reuse detection for cipher

[oif]

Use bloom filter to withstand reuse 'attack'.

Same implementation as https://github.com/shadowsocks/shadowsocks-libev/blob/v3.3.4/src/ppbloom.c

[oif]

@riobard re-implement saltfilter and support change filter entries and FPS via ldflags(but saltfilter slot count fixed). And wait https://github.com/riobard/go-bloom/pull/1 merged to modify go.mod

[oif]

pardon me, I should post the general purpose of this PR.

1. Implements a shared Bloom filter in saltfilter package and expose necessary function(Check and Test);
2. Ciphers use this shared filter directly without initialize it manually or manage it alone.

[riobard]

Please make salfilter an internal package. I don't want to export any of its members to avoid outside use.

[oif]

@riobard How about create a github.com/shadowsocks/go-shadowsocks2/internal package and put filter inside?

[riobard]

I guess that's the way to do it.

[oif]

and also suppose public errors like var ErrShortPacket = errors.New("short packet") should define in one place(maybe internal or errors)

[riobard]

What do you mean? It's already exported.

[oif]

errors declared both in aead and steam package, it’s not necessary and hard to handle outside as well

[riobard]

We will remove shadowstream package soon (I've already removed it from my fork), so there's no need to worry about that.

[oif]

@riobard OK, then just keep those declaration

[riobard]

We should add the necessary documentation.

[oif]

@riobard yep, I'll create a new PR about this soon.