Simplify algebraic method declarations. Rewrite inheritance for methods

[tnowacki]

Currently algebraic method declarations can be used for any valid child of the given type.
We should limit it to only children declared in the given class with 'children'

So

base class X1 {
  fun no(): void 
  | C1() -> void // SHOULD ERROR  
}
class C1() extends X1

base class X {
  children  = A() | B() 
  fun no(): void 
  | A() -> void // Valid!
  | C() -> void // SHOULD ERROR 
  | X _ -> void // Valid!
}
class C() extends X

Doing this will make a lot of the override rules MUCH simpler.
After this is done, the whole pass for method inheritance can probably be rewritten and simplified