* 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 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

* add simple catch-all implementation for the metadata URI interface

* include an internal function to set the URI so users can implement functionality to switch URIs

* add tests for ERC1155 metadata URI

* fix nits, mostly pointed out by linter

* convert ERC1155 metadata URI work to Solidity 0.6

* mark all non-view functions as virtual

* 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

* Update contracts/token/ERC1155/IERC1155MetadataURI.sol

Starting on Solidity v0.6.2, interfaces can now inherit. \o/

Co-authored-by: Nicolás Venturo <nicolas.venturo@gmail.com>

* Fix compile errors

* Remove URI event

* Merge MetadataCatchAll into ERC1155

* Improve documentation.

* Simplify tests

* Move tests into ERC1155 tests

* Update documentation

* Bump minimum compiler version for inteface inheritance

* Fix holder tests

* Improve setUri docs

* Fix docs generation

Co-authored-by: Alan Lu <alanlu1023@gmail.com>
Co-authored-by: Nicolás Venturo <nicolas.venturo@gmail.com>
Co-authored-by: Francisco Giordano <frangio.1@gmail.com>