Add benchmarks

[romainmenke]

• benchmark lists of variable sizes

BenchmarkListQuery_1-4        	    3000	     349864 ns/op
BenchmarkListQuery_100-4      	     500	    3534198 ns/op
BenchmarkListQuery_1K-4       	      50	   32966933 ns/op
BenchmarkListQuery_10K-4      	       5	  308862819 ns/op
BenchmarkListQuery_100K-4     	       1	 2255458560 ns/op

• benchmark objects with a variable number of fields

BenchmarkWideQuery_1_1-4      	   10000	     159626 ns/op
BenchmarkWideQuery_10_1-4     	    2000	     960218 ns/op
BenchmarkWideQuery_100_1-4    	      20	   62290987 ns/op
BenchmarkWideQuery_1K_1-4     	       1	 6342374529 ns/op
BenchmarkWideQuery_1_10-4     	   10000	     196672 ns/op
BenchmarkWideQuery_10_10-4    	    1000	    2356517 ns/op
BenchmarkWideQuery_100_10-4   	      10	  190874887 ns/op
BenchmarkWideQuery_1K_10-4    	       1	20784765533 ns/op

[coveralls]

Coverage decreased (-1.2%) to 80.999% when pulling b0a378b on mrhenry:benchmarks into 9b5a661 on graphql-go:master.

[coveralls]

Coverage decreased (-1.2%) to 81.034% when pulling 4c66ed6 on mrhenry:benchmarks into 363d9c3 on graphql-go:master.

[chris-ramon]

After merging memoizing fields support, now benchmarks results shows faster results, thanks a lot @romainmenke 🚀

(graphql)-> go test -test.bench BenchmarkListQuery
BenchmarkListQuery_1-4             10000            261764 ns/op
BenchmarkListQuery_100-4            1000           1020260 ns/op
BenchmarkListQuery_1K-4              200           6611959 ns/op
BenchmarkListQuery_10K-4              20          61487690 ns/op
BenchmarkListQuery_100K-4              3         400704058 ns/op

(graphql)-> go test -test.bench BenchmarkWideQuery
BenchmarkWideQuery_1_1-4           10000            157929 ns/op
BenchmarkWideQuery_10_1-4           3000            468788 ns/op
BenchmarkWideQuery_100_1-4           500           2769859 ns/op
BenchmarkWideQuery_1K_1-4             50          25347176 ns/op
BenchmarkWideQuery_1_10-4          10000            207748 ns/op
BenchmarkWideQuery_10_10-4          3000            584073 ns/op
BenchmarkWideQuery_100_10-4          300           4673692 ns/op
BenchmarkWideQuery_1K_10-4            30          42856935 ns/op