Single macro for defining a trait and its required interfaces

[tk3369]

See https://discourse.julialang.org/t/ann-binarytraits-jl-a-new-traits-package/37475/5?u=tk3369

In a nutshell, the ideas are:

1. do not allow anyone to add new interface functions to an existing trait.
2. check the interface implementation correct as soon as the data type is assigned to a trait

[tk3369]

The idea is to enforce consistency. The benefit is that a user would not accidentally assign and check a type, and then later change the contract. To do this, we will probably have to support a new syntax such that all interfaces are defined altogether in one place e.g.

@implement Can{Drive} by begin
    start(_)
    turn(_, direction::Float64)
    move(_, distance::Float64)
    stop(_)
end 

Then, either disallow the user from issuing another @implement Can{Drive} statement again, or else we could automatically check all assigned types against the new contract.

As for the 2nd point, it seems to be just a small convenience over two lines of code (@assign followed by @check). I don't think that is needed at the moment.

[JeffreySarnoff]

or maybe do so .. provide a seal_this_interface command/function/operation/macro which, when utilized, disallows, and until utilized, allows.