Create execution helpers

[hashedone]

Very similar to query helpers described in #28, but this time for helping execution.

The executor helpers should provide functions returning prepared CosmosMsg or SubMsg.

The executor would be a trait generated from interfaces/contracts and implemented probably directly on the Remote struct (unless some additional data would be needed there, like QuerierWrapper for queries). Single executor keeps all the exec functions callable on the contract, with signatures similar to those in mt helpers.

Similarly, as in MT helpers, it would give back the #[must_use] proxy type, which would use a builder pattern to provide metadata - I think only the funds field would be addable to create a simple message, but it should also contain the reply_on function:

fn reply_on(self, reply_on: cosmwasm_std::ReplyOn, reply_id: u64) -> ...;

Which returns another proxy object (let's call it submsg proxy for now), building SubMsg instead of Msg - any non-sub-msg fields (funds in particular) should be set before calling the reply_on.

Both execution proxy and submsg proxy should contain a fn message(self) -> ... function (not call() like in mt - call is not happening at this point), which returns a message to be passed into Response::add_message() or Response::add_submessage(). I would also opt for adding a fn schedule(self, response: Response) -> Response immediately adding the message into the response, which sometimes might be a shortcut.

• Create proxy types built from the binary message and contract address (address kept by reference), allowing to add of optional funds and reply information, building the final message to send #131
• Generate the Executor struct from the contract type #132
• Generate implementation of the contract Executor trait on the contract Remote type #133
• Generate the Executor struct from the interface trait #134
• Generate implementation of the interface Exexutor trait on the trait Remote type #135
• Generate implementation of the interface Executor trait on the contract Remote type #136