Strongly typed manifest

Created by: ascjones

Currently we dynamically read from the Cargo.toml e.g. in manifest.rs:

https://github.com/paritytech/cargo-contract/blob/master/src/workspace/manifest.rs#L372-L379

We should investigate introducing some types and use serde to deserialize into them for strongly typed access. There might be a library somewhere or we can take inspiration from https://github.com/dtolnay/trybuild/blob/master/src/manifest.rs.