Update to ink! 3.0

[Robbepop]

From chat:

I just wanted to give a short update that the release of ink! 3.0 is imminent and an internal (RC kind of) release is planned for 2020-10-07 (tomorrow). It would be really cool to see support on ink! playground for the new ink! 3.0. You can already toy around with ink! 3.0. Just fetch the current ink! master branch and get cargo-contract version 0.7 (also master branch atm). Also for cargo-contract we have been working hard to create some deploy and call commands to deploy and call contracts from the CLI. While those are super expermental it could be worth checking out - also in some branch on cargoc-contract repo but will be included in the next release.

There have been tons of changes to ink! one of which is a new metadata format that is now fully shared with Solang Solidity compiler. It is already fully supported with cargo-contract version 0.7 and should be way easier to work with than the old format.

Links:
cargo-contract branch with deploy and call commands:
https://github.com/paritytech/cargo-contract/tree/aj-contract-call

ink! 3.0:
https://github.com/paritytech/ink