Extend contract metadata with cargo metadata

Created by: ascjones

Rel https://github.com/paritytech/ink/issues/299

Cargo metadata should be used to extract the following and pass it to the ink_abi:

e.g.

• contract version
• ink! version
• rust version,
• author,
• license, etc.

If we are able to generate the main.rs (PR incoming) of the .ink/abi_gen then we can set those values directly e.g.

ink_abi::Metadata::build()
    .layout(contract_layout) // generated by ink!
    .general(contract_info) // generated by ink!
    .meta(cargo_toml_info) // injected by cargo-contract
    .finalize()