MixedIntegerLinearProgram: New generator method 'variables'

[mkoeppe]

We modify MixedIntegerLinearProgram.new_variable so that it remembers the variables.

To avoid reintroducing reference loops eliminated in #12616, we modify MIPVariable so that it no longer stores a reference to the MIP but rather to the backend, the base_ring, and the linear_functions_parent.

A new method M.variables() yields all MIPVariable instances that have been created using M.new_variable() in the order of their creation.

Depends on #20656

CC: @yuan-zhou @sagetrac-tmonteil @tscrim

Component: linear programming

Branch: u/mkoeppe/mixedintegerlinearprogram__new_generator_method__variables_

Issue created by migration from https://trac.sagemath.org/ticket/31749

[mkoeppe]

Description changed:

--- 
+++ 
@@ -1,4 +1,6 @@
 We modify `MixedIntegerLinearProgram.new_variable` so that it remembers the variables.
+
+To avoid reintroducing reference loops eliminated in #12616, we modify `MIPVariable` so that it no longer stores a reference to the MIP but rather to the `backend`, the `base_ring`, and the `linear_functions_parent`.
 
 A new method `M.variables()` yields all `MIPVariable` instances that have been created using `M.new_variable()` in the order of their creation.
 

[mkoeppe]

Dependencies: #20656

[mkoeppe]

Branch: u/mkoeppe/mixedintegerlinearprogram__new_generator_method__variables_