Implement tree-il-fold in terms of make-tree-il-folder.

Implement tree-il-fold in terms of make-tree-il-folder.

* module/language/tree-il.scm (tree-il-fold): Implement using
  make-tree-il-folder.  This is an incompatible change: there is no more
  "leaf" procedure, and tree-il-fold only works on tree-il and not
  lists.

* module/language/tree-il/analyze.scm (<tree-analysis>, analyze-tree):
  Adapt to tree-il-fold change, losing the "leaf" handler.
  (unused-variable-analysis, unused-toplevel-analysis)
  (unbound-variable-analysis, arity-analysis): Adapt to tree-analysis
  change.

* module/language/tree-il/canonicalize.scm (tree-il-any)
* module/language/tree-il/cse.scm (build-assigned-var-table)
* module/language/tree-il/peval.scm (tree-il-any, build-var-table)
  (peval): Adapt to tree-il-fold change.

* test-suite/tests/tree-il.test ("tree-il-fold"): Adapt tests for new
  interface and expectations.