No equivalent of ERC1155 standard

[yihuang]

• ERC1155 treats fungible and non-fungible tokens equally, non-fungible tokens just have one max supply.
• Metadata and token enumerations can be handled off-chain, by indexing attributes in block results.
• Approval is set to some operator for all tokens at once, no fine-granularity control over allowance.
  Implementation might assume approval for self automatically.
• Mint/burn/transfer are mixed in one set of messages, otherwise, there'll be too many message types.