Add benchmark for balances

[clearloop]

This benchmark demand is original from #123, and relate to #323, we can review weights while benchmarking.

Steps

• add benchmark command
• add benchmark for balances
  • skip kton, because it is as same as ring.
• patch docs for balances.
  • check the weight of balances

Refs

1. benchmarking command in substrate
2. benchmarking in substrate's balances
3. calcuating weights in substrate

[clearloop]

Preview

 𝝺 ./target/debug/darwinia benchmark --extrinsic transfer --pallet ring
Pallet: "ring", Extrinsic: "transfer", Steps: [], Repeat: 1
u,e,extrinsic_time,storage_root_time
1,1000,5345000,1303000
100,1000,2984000,832000
199,1000,3037000,771000
298,1000,2165000,574000
397,1000,2062000,599000
496,1000,2045000,571000
595,1000,2207000,680000
694,1000,2089000,603000
793,1000,2354000,573000
892,1000,3013000,837000
991,1000,4462000,679000
1000,2,3918000,683000
1000,101,2369000,592000
1000,200,2401000,575000
1000,299,2397000,660000
1000,398,2436000,660000
1000,497,2779000,593000
1000,596,2105000,572000
1000,695,2551000,753000
1000,794,2474000,586000
1000,893,2950000,849000
1000,992,3158000,848000
Done.

The second column extrinsic_time is the weight we calculated out exactly, and the fee can be converted from these values.