benchmarks: add 4K block size

[rfjakob]

4 KiB is interesting because it is the default
page size on Linux, and also the block size
that gocryptfs (a disk encryption tool) uses.

gocryptfs is contemplating adding AEGIS support
( rfjakob/gocryptfs#900 ).

Output now looks like this:

$ go test -bench .
goos: linux
goarch: amd64
pkg: github.com/ericlagergren/aegis
cpu: Intel(R) Core(TM) i5-3470 CPU @ 3.20GHz
BenchmarkSeal16B_128L-4   	12179401	        97.09 ns/op	 164.80 MB/s	       0 B/op	       0 allocs/op
BenchmarkOpen16B_128L-4   	11123548	       104.7 ns/op	 152.75 MB/s	       0 B/op	       0 allocs/op
BenchmarkSeal1K_128L-4    	 4735476	       253.1 ns/op	4045.21 MB/s	       0 B/op	       0 allocs/op
BenchmarkOpen1K_128L-4    	 4614565	       259.0 ns/op	3953.60 MB/s	       0 B/op	       0 allocs/op
BenchmarkSeal4K_128L-4    	 1685662	       712.3 ns/op	5750.17 MB/s	       0 B/op	       0 allocs/op
BenchmarkOpen4K_128L-4    	 1667968	       719.5 ns/op	5692.93 MB/s	       0 B/op	       0 allocs/op
BenchmarkSeal8K_128L-4    	  867411	      1327 ns/op	6175.61 MB/s	       0 B/op	       0 allocs/op
BenchmarkOpen8K_128L-4    	  858153	      1333 ns/op	6143.63 MB/s	       0 B/op	       0 allocs/op
BenchmarkSeal16K_128L-4   	  460386	      2543 ns/op	6441.87 MB/s	       0 B/op	       0 allocs/op
BenchmarkOpen16K_128L-4   	  458068	      2556 ns/op	6409.65 MB/s	       0 B/op	       0 allocs/op
BenchmarkSeal16B_256-4    	12555355	        95.04 ns/op	 168.35 MB/s	       0 B/op	       0 allocs/op
BenchmarkOpen16B_256-4    	11600582	       102.5 ns/op	 156.05 MB/s	       0 B/op	       0 allocs/op
BenchmarkSeal1K_256-4     	 3595903	       336.3 ns/op	3045.34 MB/s	       0 B/op	       0 allocs/op
BenchmarkOpen1K_256-4     	 3511225	       340.0 ns/op	3011.56 MB/s	       0 B/op	       0 allocs/op
BenchmarkSeal4K_256-4     	 1000000	      1056 ns/op	3878.99 MB/s	       0 B/op	       0 allocs/op
BenchmarkOpen4K_256-4     	 1000000	      1046 ns/op	3914.79 MB/s	       0 B/op	       0 allocs/op
BenchmarkSeal8K_256-4     	  572815	      2007 ns/op	4080.76 MB/s	       0 B/op	       0 allocs/op
BenchmarkOpen8K_256-4     	  586015	      1989 ns/op	4119.45 MB/s	       0 B/op	       0 allocs/op
BenchmarkSeal16K_256-4    	  295207	      3930 ns/op	4168.82 MB/s	       0 B/op	       0 allocs/op
BenchmarkOpen16K_256-4    	  303639	      3878 ns/op	4224.89 MB/s	       0 B/op	       0 allocs/op
PASS
ok  	github.com/ericlagergren/aegis	26.662s