Initial set of host functions

[lrettig]

• [x] get_storage
• [x] set_storage
• [ ] account_exists
• [ ] get_balance
• [ ] get_tx_context
• [ ] get_block_hash
• [ ] send
• [ ] #5

https://github.com/athenavm/athena/blob/8924ae45893a6280b6468293b826d69f0d6c2121/interface/src/lib.rs#L211-L219

[lrettig]

Note: we probably also want the ability to deploy a new template and/or contract, i.e., a create host call. I'm intentionally leaving it out of this phase because this requires deeper integration with the host. There are some non-obvious questions about how this works - how addresses are calculated, how we want to handle templates vs. program instances, how we want to vet program code before deployment, how the actual deployment process works (e.g., init code, immutable state), how we cache program code, etc. In other words, it's much more non-trivial than the other host functions here. Let's revisit this as part of the next phase.