Have attributes for contract customization

[swfsql]

The near-sdk offers various features which makes it easier and safer to build contracts, and this library should also offer them.

On Methods:

• #[contract(init)]
• #[contract(init(ignore_state))]
• #[contract(private)]
• #[contract(payable)]
• #[contract(result_serializer(borsh)] | #[contract(result_serializer(json)]
• .. ?

On parameters:

• Regular
  (read from env::input())
• #[contract(callback)] | #[contract(callback_unwrap)]
CallbackArg (read from a single env::promise_result())
• #[contract(callback_result)]
CallbackResultArg (read from a single env::promise_result() which handles err)
• #[contract(callback_vec)]
CallbackArgVec (read from all env::promise_result())
• ...?