* Erc721 basic

* Update to counter logic

* Simplify counter

* Some cleaning

* More cleaning

* Add mint and burn

* Refact counter

* Add mint test

* Add transfer-burn tests

* Add approve transfer test

* Fix error propagation

* Add tests

* Restric account 0x0

* Extend test cases

* Update Result type

Agree, it makes sense to use the Result type as defined on core.

Co-Authored-By: Gavin Wood <github@gavwood.com>

* Change Result type

* Change Result return type

* Change ErrNo to enum

* Fix ABI layout error

* Update examples/lang/erc721/src/lib.rs

Co-Authored-By: Hero Bird <robbepop@web.de>

* Update examples/lang/erc721/src/lib.rs

Co-Authored-By: Hero Bird <robbepop@web.de>

* Update examples/lang/erc721/src/lib.rs

Co-Authored-By: Hero Bird <robbepop@web.de>

* Update examples/lang/erc721/src/lib.rs

Co-Authored-By: Hero Bird <robbepop@web.de>

* Update examples/lang/erc721/src/lib.rs

Co-Authored-By: Hero Bird <robbepop@web.de>

* Update examples/lang/erc721/src/lib.rs

Co-Authored-By: Hero Bird <robbepop@web.de>

* Update examples/lang/erc721/src/lib.rs

Co-Authored-By: Hero Bird <robbepop@web.de>

* Implement to_u32 Error

* Update examples/lang/erc721/src/lib.rs

Co-Authored-By: Hero Bird <robbepop@web.de>

* Update examples/lang/erc721/src/lib.rs

Co-Authored-By: Hero Bird <robbepop@web.de>

* Update examples/lang/erc721/src/lib.rs

Co-Authored-By: Hero Bird <robbepop@web.de>

* Update examples/lang/erc721/src/lib.rs

Co-Authored-By: Hero Bird <robbepop@web.de>

* Update examples/lang/erc721/src/lib.rs

Co-Authored-By: Hero Bird <robbepop@web.de>

* Update examples/lang/erc721/src/lib.rs

Co-Authored-By: Hero Bird <robbepop@web.de>

* Update examples/lang/erc721/src/lib.rs

Co-Authored-By: Hero Bird <robbepop@web.de>

* Update examples/lang/erc721/src/lib.rs

Co-Authored-By: Hero Bird <robbepop@web.de>

* Update examples/lang/erc721/src/lib.rs

Co-Authored-By: Hero Bird <robbepop@web.de>

* Update examples/lang/erc721/src/lib.rs

Co-Authored-By: Hero Bird <robbepop@web.de>

* Update examples/lang/erc721/src/lib.rs

Co-Authored-By: Hero Bird <robbepop@web.de>

* Add tons of comments