crypto/ed25519/chainkd: add benchmarks
crypto/ed25519/chainkd: add benchmarks

Current results:
```
BenchmarkXPrvChildNonHardened-8   	   30000	     53671 ns/op	     400 B/op	
      6 allocs/op
BenchmarkXPrvChildHardened-8      	 2000000	       712 ns/op	     368 B/op	
      5 allocs/op
BenchmarkXPubChild-8              	   20000	     63540 ns/op	     848 B/op	
      8 allocs/op
BenchmarkXPrvSign-8               	   10000	    109968 ns/op	     833 B/op	
     10 allocs/op
BenchmarkXPubVerify-8             	   10000	    139911 ns/op	     352 B/op	
      3 allocs/op
```

Closes chain/chainprv#2081

chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain