Do not postprocess or generate metadata if contract unchanged

[ascjones]

Closes #949.

Checks if the original compiled .wasm file in target/ink/wasm32-unknown-unknown/release/ has changed. If it is unchanged after building, then skips the postprocessing and metadata gen iff all of the resulting contract artefacts are already there.

Note: because it must perform the build step first before determining whether the contract has changed, the first step now shows [1/*]

When there are changed to the contract:

When there have been no changes to the contract:

[cmichi]

Could you add a line to the user output, explaining that metadata was not updated because contract is unchanged?

A log::debug! when deciding to not build further would also be helpful.

[ascjones]

Could you add a line to the user output, explaining that metadata was not updated because contract is unchanged?

A log::debug! when deciding to not build further would also be helpful.

There is already a tracing::info! there for this case (we no longer use log).

Here is the message saying artifacts are unchanged: