Kill the interface {} key and put a int key

Port the tests where necessary

PASS
BenchmarkGetSeq1e3	   10000	    189165 ns/op
BenchmarkSetRnd1e3	    5000	    465189 ns/op
BenchmarkGetRnd1e3	   10000	    225564 ns/op
BenchmarkDelRnd1e3	    5000	    394045 ns/op
BenchmarkSeekRnd1e3	    5000	    319225 ns/op
BenchmarkNext1e3	  100000	     21800 ns/op
BenchmarkPrev1e3	  100000	     21491 ns/op
ok  	_/home/m/Desktop/GOLANG/gopath/b	401.547s