Refactor: Mixin semantic unification and simplification.

[Peefy]

Enhancement

Semantic

• The visibility/reference of a Mixin to its host variables can only be achieved through the Protocol (similar to the interface for property access). When the host uses a Mixin, it must satisfy the Protocol property constraints (satisfying the type partial order relationship), and it is one-way, that is Mixin is visible to the properties of the host individually, different mixin attributes are invisible to each other, and the host is invisible to the properties in the mixin.
• Mixin can only do merged incremental modification, cannot reassign the existing properties of the host, but can define the properties that the host does not have.
• Automatic merging of configuration blocks with the same name in mixins and schemas that use the merge operator :.

The KCL code is

schema Person:
    mixin [FullNameMixin]
    firstName: str
    lastName: str
    age: int
    id?: int

# protocol specifies what properties the host of the mixin must have
protocol FullNameProtocol:
    firstName: str
    lastName: str

mixin FullNameMixin for FullNameProtocol:
    fullName: str = "{} {}".format(firstName, lastName)
    
alice = Person {
    firstName = "Alice"
    lastName = "Green"
    age = 10
    id = 1
}

The output YAML is

alice:
firstName: Alice
lastName: Green
age: 10
id: 1
  fullName: Alice Green

[Peefy]

Close with #404.