modify getTypeImpl to reduce result to final implementation

[jcosborn]

Fixed getTypeImpl for second part of #5788.
This changes the behavior so that it will recurse and give the final implementation for a recursively defined type.

[Araq]

Let's hope this really doesn't require a helper Intset to detect cycles reliably.

[krux02]

This commit broke the following case:

import macros

type
  ForeverForgottenSecretIdentifier[T] = object
    value: T

  SuperMysteryType[T] = object
    value: T

  A = SuperMysteryType[int]
  B = ForeverForgottenSecretIdentifier[int]
  C = ForeverForgottenSecretIdentifier[float32]
  D = SuperMysteryType[float32]

var
  a: A
  b: B
  c: C
  d: D

macro foobar(arg: typed): untyped =
  echo arg.getTypeInst.repr
  echo arg.getTypeImpl.repr

foobar(a)
foobar(b)
foobar(c)
foobar(d)

No matter what I do, ForeverForgottenSecretIdentifier and SuperMysteryType are not recoverable by the macros module anymore. Please undo this commit.

[jcosborn]

This currently does what you want

| echo arg.getTypeInst.symbol.getImpl.repr

[krux02]

@jcosborn thanks a lot. How am I supposed to know that?