[ 💻 Clarity VM ] Define and implement interface for cross-contract calls

[obycode]

What does the interface look like when one contract calls another? Think about efficiency. This might require importing host functionality like #2, or we may be able to load all of the dependencies and link them together. Clarinet could be a good place to prototype this.

[obycode]

#116 implements static contract calls. Dynamic contract calls (with traits) are still to be implemented.

[obycode]

There is also remaining work to do on static calls to perform all of the proper checks. The PR was getting large, so I left that work for later. I will switch to print from #64, then come back to this later.

[obycode]

Dynamic contract calls are fixed in #154.

[obycode]

Done!