refactor aead implementation

[komuw]

go test -run=XXXX -bench=BenchmarkEnc -count=10 github.com/komuw/ong/cry > old.txt
go test -run=XXXX -bench=BenchmarkEnc -count=10 github.com/komuw/ong/cry > new.txt
benchstat old.txt new.txt

goos: linux
goarch: amd64
pkg: github.com/komuw/ong/cry
cpu: Intel(R) Core(TM) i5-6300U CPU @ 2.40GHz
      │   old.txt   │            new.txt            │
      │   sec/op    │   sec/op     vs base          │
Enc-4   5.713µ ± 6%   5.756µ ± 7%  ~ (p=0.853 n=10)

      │  old.txt   │            new.txt             │
      │    B/op    │    B/op     vs base            │
Enc-4   672.0 ± 0%   672.0 ± 0%  ~ (p=1.000 n=10) ¹
¹ all samples are equal

      │  old.txt   │            new.txt             │
      │ allocs/op  │ allocs/op   vs base            │
Enc-4   14.00 ± 0%   14.00 ± 0%  ~ (p=1.000 n=10) ¹
¹ all samples are equal