* adding mock contacts, test code

* adding changes to ERC721.sol per @frangio's comments on original PR #1943

* fix solhint warnings

* Update contracts/token/ERC721/ERC721.sol

Co-Authored-By: Francisco Giordano <frangio.1@gmail.com>

* same revert wording per @frangio's review suggestion

* per @frangio's feedback, changing the inline assembly to accomplish: we want to ignore the first 4 bytes of content, so we should read the length and decrease it by 4, then take the memory location and add 4 to it, then store the new length at the new memory location, then that is the new byte array that we want.

* change revert msg assembly per PR comment by @frangio

* unify revert msg in test code

* fix some failed tests, wording change

* Update contracts/token/ERC721/ERC721.sol

Co-Authored-By: Francisco Giordano <frangio.1@gmail.com>

* Update contracts/token/ERC721/ERC721.sol

Co-Authored-By: Francisco Giordano <frangio.1@gmail.com>

* fix test case, revert without reason

* fix 'ERC721ReceiverRevertsMock: Transaction rejected by receiver'

* style change per review by @frangio

* fix revert reason forwarding

* remove duplicate contracts/mocks/ERC721ReceiverRevertsMock.sol per review https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2018\#issuecomment-574381034

* Add changelog entry

* Fix tests

* Make tests more clear

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