generally disallow recursive structural types, check proc param types

[metagn]

fixes #5631, fixes #8938, fixes #18855, fixes #19271, fixes #23885, fixes #24877

isTupleRecursive, previously only called to give an error for illegal recursions for:

• tuple fields
• types declared in type sections
• explicitly instantiated generic types

did not check for recursions in proc types. It now does, meaning proc types now need a nominal type layer to recurse over themselves. It is renamed to isRecursiveStructuralType to better reflect what it does, it is different from a recursive type that cannot exist due to a lack of pointer indirection which is possible for nominal types.

It is now also called to check the param/return types of procs, similar to how tuple field types are checked. Pointer indirection checks are not needed since procs are pointers.

I wondered if this would lead to a slowdown in the compiler but since it only skips structural types it shouldn't take too many iterations, not to mention only proc types are newly considered and aren't that common. But maybe something in the implementation could be inefficient, like the cycle detector using an IntSet.

Note: The name isRecursiveStructuralType is not exactly correct because it still checks for distinct types. If it didn't, then the compiler would accept this:

type
  A = distinct B
  B = ref A

But this breaks when attempting to write var x: A. However this is not the case for:

type
  A = object
    x: B
  B = ref A

So a better description would be "types that are structural on the backend".

A future step to deal with #14015 and #23224 might be to check the arguments of tyGenericInst as well but I don't know if this makes perfect sense.

[Araq]

Definitely improves the situation but for the future we should remove the restriction that recursive types must go through a nominal type and instead simply accept these definitions and make sigmatch etc check for endless recursions.

[github-actions]

Thanks for your hard work on this PR!
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