fix[next]: lower function definitions to let bindings

[tehrengruber]

InlineFundefs replaced every SymRef matching a program-level function definition, regardless of any binder of the same name in between. A lambda parameter named like a field operator was therefore silently replaced by that function, producing an ill-typed program (e.g. an AssertionError in type_synthesizer._canonicalize_nb_fields).

Instead of substituting references, all function definitions are now bound in nested lets wrapping every statement expression, and Program.function_definitions is cleared. Regular scoping rules then apply, i.e. an inner binder shadows the function definition, and unused bindings are removed by dead code elimination, which makes the separate prune_unreferenced_fundefs pass obsolete. The lets are nested in topological order since function definitions may call each other.

Follow-up to #2833 (comment), where tuple-comprehension targets made this easy to hit.

AI disclaimer: This PR has been created with the help of AI tools. The code has been reviewed, but tests were only briefly sighted.

[tehrengruber]

Since the PR does not inline every function right away it might lead to some problems in the dace backend. We might want to force inline functions for dace and maybe even gtfn. The approach in this PR generally better since we start with a smaller IR (after the first inline lambda which removes unused functions) that we can optimize before inlining later. We force inline all lambdas anyway right now.