Simple benchmark

[qnfm]

go test -run='^$' -bench='AESGEM256' -benchmem -benchtime=3s                 1 ✘
goos: linux
goarch: amd64
pkg: trailofbits.com/aes-gem
cpu: 11th Gen Intel(R) Core(TM) i7-1185G7 @ 3.00GHz
BenchmarkAESGEM256Seal4K-8     	    7242	    503826 ns/op	   8.13 MB/s	    1216 B/op	      13 allocs/op
BenchmarkAESGEM256Seal16K-8    	    1792	   2008411 ns/op	   8.16 MB/s	    1216 B/op	      13 allocs/op
BenchmarkAESGEM256Seal64K-8    	     451	   8060555 ns/op	   8.13 MB/s	    1216 B/op	      13 allocs/op
BenchmarkAESGEM256Seal256K-8   	     100	  32342519 ns/op	   8.11 MB/s	    1216 B/op	      13 allocs/op
BenchmarkAESGEM256Seal1M-8     	      25	 128861683 ns/op	   8.14 MB/s	    1216 B/op	      13 allocs/op
BenchmarkAESGEM256Open4K-8     	    7089	    512756 ns/op	   7.99 MB/s	    1216 B/op	      13 allocs/op
BenchmarkAESGEM256Open16K-8    	    1761	   2828967 ns/op	   5.79 MB/s	    1216 B/op	      13 allocs/op
BenchmarkAESGEM256Open64K-8    	     195	  18325310 ns/op	   3.58 MB/s	    1216 B/op	      13 allocs/op
BenchmarkAESGEM256Open256K-8   	      50	  60257991 ns/op	   4.35 MB/s	    1216 B/op	      13 allocs/op
BenchmarkAESGEM256Open1M-8     	      19	 179219557 ns/op	   5.85 MB/s	    1216 B/op	      13 allocs/op
PASS
ok  	trailofbits.com/aes-gem	49.132s

[CLAassistant]

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[qnfm]

I'll mark it as draft in case you are interested and want cherry pick/split this pr