key: add benchmark for creating key.Key on built-in types
key: add benchmark for creating key.Key on built-in types

benchmark result:
BenchmarkBuiltInType/string-8         	30000000	        47.9 ns/op	      16 B/op	       1 allocs/op
BenchmarkBuiltInType/int8-8           	30000000	        47.6 ns/op	      16 B/op	       1 allocs/op
BenchmarkBuiltInType/int16-8          	30000000	        49.6 ns/op	      16 B/op	       1 allocs/op
BenchmarkBuiltInType/int32-8          	30000000	        48.6 ns/op	      16 B/op	       1 allocs/op
BenchmarkBuiltInType/int64-8          	30000000	        49.1 ns/op	      16 B/op	       1 allocs/op
BenchmarkBuiltInType/uint8-8          	30000000	        48.5 ns/op	      16 B/op	       1 allocs/op
BenchmarkBuiltInType/uint16-8         	30000000	        46.7 ns/op	      16 B/op	       1 allocs/op
BenchmarkBuiltInType/uint32-8         	30000000	        47.3 ns/op	      16 B/op	       1 allocs/op
BenchmarkBuiltInType/uint64-8         	30000000	        46.1 ns/op	      16 B/op	       1 allocs/op
BenchmarkBuiltInType/float32-8        	30000000	        48.4 ns/op	      16 B/op	       1 allocs/op
BenchmarkBuiltInType/float64-8        	30000000	        48.4 ns/op	      16 B/op	       1 allocs/op
BenchmarkBuiltInType/bool-8           	30000000	        48.3 ns/op	      16 B/op	       1 allocs/op

Change-Id: I6f64e9102c0be330f8a58cb315996a2318c0b370

aristanetworks/goarista,aristanetworks/goarista