[crypto/ed25519] Preallocate batch size for verification

[patrick-ogrady]

Related: #645

hdevalence/ed25519consensus#16

Previous

BenchmarkConsensusBatchAddVerify/1-12   	   20367	     58998 ns/op	    5648 B/op	       9 allocs/op
BenchmarkConsensusBatchAddVerify/4-12   	    9326	    130121 ns/op	   17232 B/op	      17 allocs/op
BenchmarkConsensusBatchAddVerify/16-12  	    2929	    420688 ns/op	   69264 B/op	      43 allocs/op
BenchmarkConsensusBatchAddVerify/64-12  	     775	   1545830 ns/op	  253649 B/op	     141 allocs/op
BenchmarkConsensusBatchAddVerify/128-12 	     394	   3058001 ns/op	  496850 B/op	     270 allocs/op
BenchmarkConsensusBatchAddVerify/512-12 	      97	  12106657 ns/op	 1927155 B/op	    1040 allocs/op
BenchmarkConsensusBatchAddVerify/1024-12         	      50	  24355565 ns/op	 3812816 B/op	    2065 allocs/op
BenchmarkConsensusBatchAddVerify/4096-12         	      10	 100681646 ns/op	15424976 B/op	    8213 allocs/op
BenchmarkConsensusBatchAddVerify/16384-12        	       3	 448992083 ns/op	63077872 B/op	   32795 allocs/op
BenchmarkConsensusBatchVerify/1-12               	   20228	     59853 ns/op	    5552 B/op	       7 allocs/op
BenchmarkConsensusBatchVerify/4-12               	    9436	    130756 ns/op	   16704 B/op	      10 allocs/op
BenchmarkConsensusBatchVerify/16-12              	    2968	    412512 ns/op	   67008 B/op	      22 allocs/op
BenchmarkConsensusBatchVerify/64-12              	     782	   1531430 ns/op	  243969 B/op	      70 allocs/op
BenchmarkConsensusBatchVerify/128-12             	     392	   3096470 ns/op	  476929 B/op	     134 allocs/op
BenchmarkConsensusBatchVerify/512-12             	      99	  12071803 ns/op	 1845765 B/op	     518 allocs/op
BenchmarkConsensusBatchVerify/1024-12            	      50	  24384858 ns/op	 3657735 B/op	    1030 allocs/op
BenchmarkConsensusBatchVerify/4096-12            	      12	  99508948 ns/op	14532608 B/op	    4102 allocs/op
BenchmarkConsensusBatchVerify/16384-12           	       3	 449900194 ns/op	57982976 B/op	   16390 allocs/op

PR (99% reduction in allocs)

BenchmarkConsensusBatchAddVerify/1-12   	   20077	     60035 ns/op	    5728 B/op	       8 allocs/op
BenchmarkConsensusBatchAddVerify/4-12   	    9250	    131768 ns/op	   17312 B/op	       8 allocs/op
BenchmarkConsensusBatchAddVerify/16-12  	    2857	    417964 ns/op	   69216 B/op	       8 allocs/op
BenchmarkConsensusBatchAddVerify/64-12  	     759	   1599658 ns/op	  252832 B/op	       8 allocs/op
BenchmarkConsensusBatchAddVerify/128-12 	     380	   3107973 ns/op	  494626 B/op	       8 allocs/op
BenchmarkConsensusBatchAddVerify/512-12 	      99	  12248031 ns/op	 1919529 B/op	       8 allocs/op
BenchmarkConsensusBatchAddVerify/1024-12         	      49	  24498628 ns/op	 3797029 B/op	       8 allocs/op
BenchmarkConsensusBatchAddVerify/4096-12         	      10	 101433183 ns/op	15065129 B/op	       8 allocs/op
BenchmarkConsensusBatchAddVerify/16384-12        	       3	 452705625 ns/op	60096544 B/op	       8 allocs/op
BenchmarkConsensusBatchVerify/1-12               	   19965	     60094 ns/op	    5552 B/op	       7 allocs/op
BenchmarkConsensusBatchVerify/4-12               	    8989	    131767 ns/op	   16608 B/op	       7 allocs/op
BenchmarkConsensusBatchVerify/16-12              	    2839	    418850 ns/op	   66528 B/op	       7 allocs/op
BenchmarkConsensusBatchVerify/64-12              	     763	   1565576 ns/op	  241953 B/op	       7 allocs/op
BenchmarkConsensusBatchVerify/128-12             	     388	   3068891 ns/op	  472865 B/op	       7 allocs/op
BenchmarkConsensusBatchVerify/512-12             	      98	  12609815 ns/op	 1829415 B/op	       7 allocs/op
BenchmarkConsensusBatchVerify/1024-12            	      46	  24334647 ns/op	 3624998 B/op	       7 allocs/op
BenchmarkConsensusBatchVerify/4096-12            	      12	  99872802 ns/op	14401584 B/op	       7 allocs/op
BenchmarkConsensusBatchVerify/16384-12           	       3	 450050639 ns/op	57458720 B/op	       7 allocs/op