Validate a block or operation the minimal number of times when producing the object.

### What is wrong?

We should ensure that we only validate a block or operation the minimal number of times when producing one of these objects. For example, when making a block we want to assemble the block, validate it and then broadcast. We do _not_ want to validate the block a second time when "importing" the block we just produced.

### How can it be fixed

Check what we do now... this could involve making new types or altering existing function signatures to track e.g. "validated block" vs "non-validated block"