Support recursive type descriptors

[gldubc]

Summary

• add recursive descriptor nodes and unfolding support in descriptor operations
• add recursive descriptor tests
• keep recursive map-domain if_set lazy to avoid infinite unfolding during construction

Validation

• make compile
• make test_stdlib TEST_FILES="module/types/recursive_test.exs"
• make test_stdlib TEST_FILES="module/types/descr_test.exs"

Some extra information

This PR adds a new main data-structure: a type node, which essentially encodes a way to unfold a recursive type and lives next to the descr structure. For instance, int_list = {integer(), int_list} or nil will be encoded using a node that can produce the Descr representing {integer(), int_list} or nil.

Emptiness detection is done using a simple algorith, with no memoization, that keeps track of which bdds have been encountered before. A memoizing algorith is possible, and could be implemented too if performances becomes an issue, notably when instantiating polymorphic types.

Limitation (edit: solved by f889551)

Currently, recursive types don't support opt_*. An easy way to loop: build

X = nil | {integer(), X}

and try to compute opt_intersection(<node X>, <node X>).

<node X> unfolds to the descr

 nil | {integer(), <node X>}

So when opt_intersection/2 intersects tuple fields individually, it eventually asks:

opt_intersection(<node X>, <node X>)

which unfolds to the same thing again.

Assisted-by: Codex:GPT-5 (writing tests)

[josevalim]

Why do we need this?

[gldubc]

To avoid the recursive type X = %{atom => if_set(X)} looping due to if_set(X) being like a union X or not_set()

[gldubc]

@josevalim f889551 implements in opt operations the tracking that a set operations(intersection, union, difference) has already been tried on two bdds. This is threaded in seen. This solves the limitation of the PR.

Assisted-by: Fable (to test all the possible loop paths in the opt operations)