Add GetAttribute trait

[mtshiba]

By nature, Python objects do not determine their instance attributes until runtime.
For example, a pandas.DataFrame reads a file at runtime and generates attributes.

Currently Erg provides __getattribute__ to access such dynamically generated attributes, but type information is lost.

By adding the GetAttribute trait, we allow customization of dot access. Of course, this is not recommended, but may be necessary for Python API static typing.

"""
This trait permits override of dot access.
Unless you are a developer of a library, you should not use this.
"""
GetAttribute = Trait {
    """
    When attribute resolution fails, Erg attempts to derive the attribute type using this function if the target type implements Dot.
    """
    GetAttributeType: (Attr: Str) -> Type or NoneType
}

C = Class()
C|<: GetAttribute|.
    GetAttributeType Attr =
        match Attr:
            "foo" -> Int
            _ -> None
C.
    bar = 1
    __getattribute__(self, attr) =
        if attr == "foo", do:
            C.__getattribute__::return 1
        super().__getattribute__(attr)

c = C.new()
assert c.bar == 1
assert c.foo == 1

Once GetAttribute and compile-time functions are implemented, it will be possible:

# compile-time function
FrameShape Path =
    src = open(Path).read()
    ...
    columns = ...
    index = ...
    (columns, index)

DataFrame = (Columns: {Str: Type}, Index: {Str: Type}) -> Type
DataFrame(Columns, _)|<: GetAttribute|.
    GetAttributeType Attr = Columns.get Attr
DataFrame(_, _).
    read_csv!: (|S: Str|(path: {P}) => DataFrame *FrameShape(P)) \
        and ((path: Str) => DataFrame _, _)