Implement `Env` via wasm imports, not serde json

[ethanfrey]

Current State

Currently Env is a nested json struct with 5 concrete fields (BlockInfo and ContractInfo).

This is very easy to construct and pass in tests, but it is a bunch of json parsing on every call, which is not ideal. It could also be more compile-time type-safe (not just relying on json parsing for that). This is referenced in each contract as a concrete type: env: Env.

Proposal

1. Rename Env to MockEnv and we use this just for the tests.
2. Make trait Env with accessors, which is implemented by MockEnv
3. Create an ExternalEnv, which provides the same methods but makes calls wasm exports to get the values when needed
4. handle/init/query all now take env: &dyn Env for the argument. And call eg. env.contract_address() instead of env.contract.address.clone()

Here is the trait I am thinking of, which is just flattened accessors from the current Env type.

pub trait Env {
  fn block_height(&self) -> u64;
  fn block_time(&self) -> u64;
  fn block_time_nanos(&self) -> u64;
  fn chain_id(&self) -> String;
  fn contract_address(&self) -> HumanAddress;
}

Proposing this for 0.14. Either we do it then, or we reject this.

[webmaster128]

In 3. we could also implement lazy deserializing instead of individual calls to the host. So you would always pass the data into the contract but only deserialize when the first field is accesses.

However, I don't think this is a priority right now. Let's get back to this if we have a better reason to do so.