Add toString() and toJSON() to H128, H160, H256, H512

[joojis]

• toString(): string
  The string is a hex string without 0x prefix.

• toJSON(): string
  The string is a hex string with 0x prefix. CodeChain JSON-RPC server only accepts 0x prefixed hex string.

[joojis]

toJSON(): #40

[joojis]

toString() PR has been merged: #39

[joojis]

Landed.