Rewrite benchmarks

[mai7star]

• Rewrite benchmarks with Call json.Marshal() directly in threadUnsafeSet[T].MarshalJSON() #180 style.

Sample

BenchmarkNew/1_Safe-10             	100000000	        10.71 ns/op	       0 B/op	       0 allocs/op
BenchmarkNew/1_Unsafe-10           	100000000	        10.69 ns/op	       0 B/op	       0 allocs/op
BenchmarkNew/10_Safe-10            	 9977494	       120.4 ns/op	     328 B/op	       3 allocs/op
BenchmarkNew/10_Unsafe-10          	10061940	       120.5 ns/op	     328 B/op	       3 allocs/op
BenchmarkNew/100_Safe-10           	 1338205	       858.4 ns/op	    2344 B/op	       3 allocs/op
BenchmarkNew/100_Unsafe-10         	 1391928	       897.8 ns/op	    2344 B/op	       3 allocs/op
BenchmarkAdd/1_Safe-10             	135565047	         8.909 ns/op	       0 B/op	       0 allocs/op
BenchmarkAdd/1_Unsafe-10           	223498820	         5.520 ns/op	       0 B/op	       0 allocs/op
BenchmarkAdd/10_Safe-10            	13076287	        95.78 ns/op	       0 B/op	       0 allocs/op
BenchmarkAdd/10_Unsafe-10          	24312968	        49.42 ns/op	       0 B/op	       0 allocs/op
BenchmarkAdd/100_Safe-10           	 1259056	       949.5 ns/op	       0 B/op	       0 allocs/op
BenchmarkAdd/100_Unsafe-10         	 2350886	       510.2 ns/op	       0 B/op	       0 allocs/op
BenchmarkAppend/1_Safe-10          	124188933	         9.629 ns/op	       0 B/op	       0 allocs/op
BenchmarkAppend/1_Unsafe-10        	204097729	         5.627 ns/op	       0 B/op	       0 allocs/op
BenchmarkAppend/10_Safe-10         	26909272	        44.44 ns/op	       0 B/op	       0 allocs/op
BenchmarkAppend/10_Unsafe-10       	30450057	        39.86 ns/op	       0 B/op	       0 allocs/op
BenchmarkAppend/100_Safe-10        	 2480677	       451.2 ns/op	       0 B/op	       0 allocs/op
BenchmarkAppend/100_Unsafe-10      	 2660997	       440.9 ns/op	       0 B/op	       0 allocs/op
BenchmarkAppendFrom/1_Safe-10      	31345197	        38.41 ns/op	       0 B/op	       0 allocs/op

...snip