Improve static error for incorrect arities of subclass constructors

[usaoc]

Classes with private immutable fields are currently required to have custom constructors. For example,

#lang rhombus
class Secret(private _secret)

this triggers “class needs a custom constructor to initialize private immutable fields”, while

#lang rhombus
class Secret(private _secret):
  constructor (secr):
    super(secr)

this works. However, this requirement doesn’t seem to properly propagate. For example,

#lang rhombus
class Secret(private _secret):
  nonfinal
  constructor (secr):
    super(secr)

class TopSecret():
  extends Secret

this does not trigger “class needs a custom constructor to initialize private immutable fields”, but it simply cannot work.

[usaoc]

This seems to be a more general problem, because the following program

#lang rhombus
class Secret(secret):
  nonfinal
  constructor ():
    super("not secret")

class TopSecret():
  extends Secret

doesn’t trigger any static error, either. Actually, this is a documented behavior (keeping in mind that private fields are not visible to subclasses):

If a superclass has a custom constructor, the default constructor of a subclass assumes that the superclass constructor accepts the same argument as the default superclass constructor.

However, I think a best-effort static error can still help a lot in such cases.