feat: add ContainsOne method

[vladvalkov]

This PR solves issue #118 by adding ContainsOne method, which does not cause additional allocations.
Also added some benchmarks to compare performance in different cases.

[deckarep]

@vladvalkov - hey thanks for this! I forgot that this method was talked about as an improvement to mitigate allocations. Thanks for the holiday present!

LGTM =)

[deckarep]

Nice win, thanks for the thorough benchmarking!

BenchmarkContainsComparison1Unsafe/Contains-12         	61960590	        19.25 ns/op	       8 B/op	       1 allocs/op
BenchmarkContainsComparison1Unsafe/Contains_slice-12   	160231255	         7.539 ns/op	       0 B/op	       0 allocs/op
BenchmarkContainsComparison1Unsafe/ContainsOne-12      	221081941	         5.431 ns/op	       0 B/op	       0 allocs/op
BenchmarkContainsComparison1Safe/Contains-12           	38062934	        31.10 ns/op	       8 B/op	       1 allocs/op
BenchmarkContainsComparison1Safe/Contains_slice-12     	59746304	        20.06 ns/op	       0 B/op	       0 allocs/op
BenchmarkContainsComparison1Safe/ContainsOne-12        	72793695	        16.53 ns/op	       0 B/op	       0 allocs/op
BenchmarkContainsComparison10Unsafe/Contains-12        	 5283321	       219.6 ns/op	      80 B/op	      10 allocs/op
BenchmarkContainsComparison10Unsafe/Contains_slice-12  	11263368	       106.4 ns/op	       0 B/op	       0 allocs/op
BenchmarkContainsComparison10Unsafe/ContainsOne-12     	14033839	        85.38 ns/op	       0 B/op	       0 allocs/op
BenchmarkContainsComparison10Safe/Contains-12          	 3539056	       334.7 ns/op	      80 B/op	      10 allocs/op
BenchmarkContainsComparison10Safe/Contains_slice-12    	 5102409	       235.1 ns/op	       0 B/op	       0 allocs/op
BenchmarkContainsComparison10Safe/ContainsOne-12       	 6373026	       188.0 ns/op	       0 B/op	       0 allocs/op
BenchmarkContainsComparison100Unsafe/Contains-12       	  405156	      2643 ns/op	     800 B/op	     100 allocs/op
BenchmarkContainsComparison100Unsafe/Contains_slice-12 	  934239	      1125 ns/op	       0 B/op	       0 allocs/op
BenchmarkContainsComparison100Unsafe/ContainsOne-12    	 1321592	       908.6 ns/op	       0 B/op	       0 allocs/op
BenchmarkContainsComparison100Safe/Contains-12         	  329816	      3508 ns/op	     800 B/op	     100 allocs/op
BenchmarkContainsComparison100Safe/Contains_slice-12   	  492885	      2419 ns/op	       0 B/op	       0 allocs/op
BenchmarkContainsComparison100Safe/ContainsOne-12      	  618212	      1949 ns/op	       0 B/op	       0 allocs/op