perf: Add AES key caching

[bodgit]

Before:

$ go test -v -run '^$' -bench='AES'
goos: darwin
goarch: amd64
pkg: github.com/bodgit/sevenzip
cpu: Intel(R) Core(TM) i9-8950HK CPU @ 2.90GHz
BenchmarkAES7z
BenchmarkAES7z-12    	       2	 533112205 ns/op
PASS
ok  	github.com/bodgit/sevenzip	1.808s

After:

$ go test -v -run '^$' -bench='AES'
goos: darwin
goarch: amd64
pkg: github.com/bodgit/sevenzip
cpu: Intel(R) Core(TM) i9-8950HK CPU @ 2.90GHz
BenchmarkAES7z
BenchmarkAES7z-12    	    1167	   1063848 ns/op
PASS
ok  	github.com/bodgit/sevenzip	2.427s

Fixes #187

[coveralls]

coverage: 75.106% (+0.08%) from 75.024%
when pulling df6194b on aes7z-performance
into 31f77ef on main.