Better chacha20 implementation

[xqdoo00o]

Replace by lib github.com/aead/chacha20

[riobard]

Why is it "better"?

[xqdoo00o]

OK,I test it on amd64 linux machine.

go test -test.bench=BenchmarkChaCha20_1k 
pkg: github.com/Yawning/chacha20
BenchmarkChaCha20_1k 	 1000000	      1404 ns/op	 729.19 MB/s

go test -test.bench=BenchmarkChaCha20_1K
pkg: github.com/aead/chacha20
BenchmarkChaCha20_1K 	 1000000	      1148 ns/op	 891.70 MB/s

After lots of testing, I think the encrypt perf has 20% increase in general, and iperf test also confirm this.
(maybe more readability code for me ...)

[riobard]

Great! Thanks for the PR!