Detect cycles in exceptions

[denismerigoux]

Right now the algorithm that builds the default tree from the list of rules defining a variable expects that there are no cycles in the label and exception declaration.

catala/src/catala/desugared/desugared_to_scope.ml

Lines 25 to 33 in 839310d

	(** Transforms a flat list of rules into a tree, taking into account the priorities declared between
	rules
	{e Invariant:} there are no exceptions cycles
	{e Invariant:} there are no dandling exception pointers in the rules *)
	let rec def_map_to_tree (def_info : Ast.ScopeDef.t)
	(is_def_func : Scopelang.Ast.typ Pos.marked option) (def : Ast.rule Ast.RuleMap.t) :
	rule_tree list =

This requirement is not enforced, so we should built a dependency graph whose vertices are label and edges are exception, and check for cycles in this graph for each scope variable (defined by a list of rules).

The code for this graph checking could live in src/catala/desugared/dependency.ml.

[denismerigoux]

When fixing this issue, test it on this case:

catala/tests/test_exception/exceptions_cycle.catala

Lines 1 to 19 in ea117d1

	@Test@
	/*
	new scope A:
	param x content int
	scope A:
	label base_x
	exception exception_exception_x
	def x := 0
	label exception_x
	exception base_x
	def x := 1
	label exception_exception_x
	exception exception_x
	def x := 2
	*/

[denismerigoux]

Fixed by 6787d5b