Misc features and and quality of life improvements motivated by Op testing

[intoverflow]

• [x] Create cache directories if they do not already exist
• [ ] Allow verify of receipt from disk

[intoverflow]

Addresses https://github.com/risc0/zeth/issues/40 by creating missing directories

[intoverflow]

Let's save "Allow verify of receipt from disk" for a later PR, as it is unclear that this is actually needed