Improve programs testing

[iFrostizz]

We need to improve programs testing.
As raised by this issue #1036, there are multiple drawbacks with the current testing pattern:

• We need to keep track of an environment variable to locate the WASM binary
• Steps are written by hand, without any type-safe guarantees such as with bindings, see Add bindings feature to programs #1009

As a first proposal, I wanted to show an example of what could be done to solve this.
We could base ourselves on the bindings feature to create a test_bindings one. Here is what we would do:

• Wrap programs in a module called program (or wathever)
• Annotate this module with an attribute macro, such as #[wasmlanche_program]
• This macro will expand to a constant that locates the binary based on the location of the program as well as cargo settings
• Functions annotated with the #[public] attribute expands to a function that passes a reference to the simulator as the first arg, then follow Context and all the rest. The inner of the function is just what we currently do with Step manually (see 3788373#diff-c0fe3c9010bcd916d3acf32027541e1943e9ebe8650d5901e536157ed0a590a3R138-R145).

So it could expand to this:

#[cfg(not(feature = "bindings"))]
use wasmlanche_sdk::Context;
use wasmlanche_sdk::{public, state_keys, types::Address, Gas, Program};

mod program {
    use super::*;

    #[state_keys]
    pub enum StateKeys {
        /// The count of this program. Key prefix 0x0 + address
        Counter(Address),
    }

    type Count = u64;

    /// Increments the count at the address by the amount.
    #[cfg(feature = "test_bindings")]
    pub fn inc(
        simulator: Client<W: std::io::Write, R: Iterator<Item = Result<StepResponse, StepError>>>,
        context: Context<StateKeys>,
        to: Address,
        amount: Count,
    ) -> Result<bool, StepError> {
        use simulator::{Client, Step, StepError, StepResponse};

        simulator
            .run_step(
                owner_key,
                &Step {
                    endpoint: Endpoint::Execute,
                    method: "inc".into(),
                    max_units: 1000000,
                    params: vec![counter_id.into(), to, amount],
                },
            )?
            .result
            .response::<bool>() // TODO figure out error handling
            .unwrap()
    }
}