Implement `BuildMode`

Created by: cmichi

Closes https://github.com/paritytech/cargo-contract/issues/284.

Kinda blocked until we have a release of ink! 3.0.0-rc4, since the ink_env/ink-debug feature is newer than the currently released 3.0.0-rc3. The tests are failing because they use the published ink! crates.

This PR adds a new CLI flag --release, which mirrors the behavior of cargo ‒ compiling the artifact in release mode is not done by default.

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