Write an RFC for traits as supertraits for abis

[mohammadfawaz]

Syntax:

trait MyTrait {
    // Interface
}

abi MyContract : MyTrait {
    // Interface
} {
    // Default-implemented methods 
}

Semantics:

• The implementation of the abi is allowed to use the methods from MyTrait.
• The methods from MyTrait are not actual contract methods... they are only callable from inside the library/contract.