Implement inheritance

[LFsWang]

Add support to inheritance

Parser

• New keywords

• [x] abstract
• [x] is
• [x] override
• [x] virtual
• [x] interface

• structure

• [ ] identifier-path
• [x] inheritance-specifier
• [x] override-specifier

• Add new ability on AST for inheritance

Analysis

• Function Overriding

• [x] Base functions can be overridden by inheriting contracts to change their behavior if they are marked as virtual
• [x] The overriding function must then use the override keyword in the function header
• [x] The overriding function may only change the visibility of the overridden function from external to public
• [x] nonpayable can be overridden by view and pure
• [x] view can be overridden by pure
• [x] payable is an exception and cannot be changed to any other mutability.
• [ ] For multiple inheritance, the most derived base contracts that define the same function must be specified explicitly after the override
• [x] Functions with the private visibility cannot be virtual
• [ ] Public state variables can override external functions if the parameter and return types of the function matches the getter function of the variable:

• Modifier Overriding (The details is as same as Function Overriding)

• [ ] modifier support.

• Constructors

• Multiple Inheritance and Linearization

• [x] C3 Linearization (Build inheritance graph)
• [ ] Inheriting Different Kinds of Members of the Same Name

• Abstract Contracts

• Interfaces

• [x] They cannot inherit from other contracts, but they can inherit from other interfaces.

• [x] All declared functions must be external

• [x] They cannot declare a constructor.

• [x] They cannot declare state variables.

• [x] All functions declared in interfaces are implicitly virtual

• contract injector