* Initial ERC1155 implementation with some tests (#1803)
* Initial ERC1155 implementation with some tests
* Remove mocked isERC1155TokenReceiver
* Revert reason edit nit
* Remove parameters associated with isERC1155TokenReceiver call
* Add tests for approvals and single transfers
* Add tests for transferring to contracts
* Add tests for batch transfers
* Make expectEvent.inTransaction tests async
* Renamed "owner" to "account" and "holder"
* Document unspecified balanceOfBatch reversion on zero behavior
* Ensure accounts can't set their own operator status
* Specify descriptive messages for underflow errors
* Bring SafeMath.add calls in line with OZ style
* Explicitly prevent _burn on the zero account
* Implement batch minting/burning
* Refactored operator approval check into isApprovedForAll calls
* Renamed ERC1155TokenReceiver to ERC1155Receiver
* Added ERC1155Holder
* Fix lint issues
* Migrate tests to @openzeppelin/test-environment
* Port ERC 1155 branch to Solidity 0.6 (and current master) (#2130)
* port ERC1155 to Solidity 0.6
* make ERC1155 constructor more similar to ERC721 one
* also migrate mock contracts to Solidity 0.6
* mark all non-view functions as virtual
Co-authored-by: Alan Lu <alanlu1023@gmail.com>
Co-authored-by: Nicolás Venturo <nicolas.venturo@gmail.com>
Co-authored-by: Robert Kaiser <kairo@kairo.at>
