Tree-il post-order rewriter no longer destructive

Tree-il post-order rewriter no longer destructive

* module/language/tree-il.scm (pre-post-order): New helper, like
  pre-order! and post-order! but not destructive.
  (post-order): Implement in terms of pre-post-order, and rename from
  post-order!.

* module/ice-9/compile-psyntax.scm (squeeze-tree-il):
* module/language/tree-il/canonicalize.scm (canonicalize):
* module/language/tree-il/fix-letrec.scm (fix-letrec):
* module/language/tree-il/primitives.scm (resolve-primitives): Use
  post-order, and rename from the destructive
  variants (squeeze-tree-il!, canonicalize!, etc).  Adapt callers.

* test-suite/tests/tree-il.test (strip-source): Adapt to post-order.

* test-suite/tests/cse.test:
* test-suite/tests/peval.test:
* module/language/tree-il/optimize.scm: Adapt callers.