Implement Generic Type Method Declaration

[Alexius-Huang]

What?
Currently method declaration for generic type seems haven't implemented yet.

Why?
For instance, we cannot declare method for types like List<element>

How?
Generic type method of specific type:

def List<Num>.<method-name>(<params: type>): <output> do
  ...
end

Generic type method of generic type:

def List<element: T>.<method-name>(<params: type | T>): <output | T> do
  ...
end