s2: Use sorted search for index

[klauspost]

cpu: AMD Ryzen 9 3950X 16-Core Processor
BenchmarkIndexFind
BenchmarkIndexFind/blocks-1-32         	194769370	         6.180 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-2-32         	199258790	         6.057 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-4-32         	204297220	         5.879 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-8-32         	190476280	         6.321 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-16-32        	158611744	         7.537 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-32-32        	100000000	        10.04 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-64-32        	79986668	        14.90 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-128-32       	74996250	        14.95 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-256-32       	19506928	        60.93 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-512-32       	 8450894	       142.1 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-1024-32      	 3858536	       307.3 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-2048-32      	 2556153	       470.4 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-4096-32      	  840476	      1457 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-8192-32      	  571398	      2091 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-16384-32     	  195525	      6022 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-32768-32     	  138596	      8665 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-65535-32     	   86328	     13993 ns/op	       0 B/op	       0 allocs/op

after:

cpu: AMD Ryzen 9 3950X 16-Core Processor
BenchmarkIndexFind
BenchmarkIndexFind/blocks-1-32         	195354979	         6.054 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-2-32         	193981434	         6.140 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-4-32         	201894782	         6.023 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-8-32         	180996812	         6.978 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-16-32        	148147708	         8.016 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-32-32        	100000000	        10.43 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-64-32        	75000000	        15.95 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-128-32       	75000000	        15.59 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-256-32       	49979174	        22.83 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-512-32       	46302012	        25.00 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-1024-32      	44444937	        27.43 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-2048-32      	38709552	        29.62 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-4096-32      	37005741	        31.86 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-8192-32      	34297766	        34.43 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-16384-32     	30763077	        36.89 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-32768-32     	29656209	        39.45 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-65535-32     	27980180	        42.53 ns/op	       0 B/op	       0 allocs/op

Fixes #554