…tests updated and commented.

Otherwise compiling again without changes returns status 0
and no errors.

fixes issues found in the incremental builds, via comparing incremental
vs. clean builds on example projects like prelude

Each of these caused a stale but successful build:

- Re-exports via unqualified imports (`module B (module A) where
  import A`, the common prelude pattern) were not matched against
  downstream usage, so changes or removals flowing through such
  re-exports failed to rebuild consumers.
- Newly added re-exports were absent from the externs diff, so a name
  newly brought into scope could not trigger a rebuild of a module it
  made ambiguous.
- do/ado notation and unary minus desugar into references to bind,
  discard, map, apply, pure and negate that are not present in the
  parsed module, so changes to those names went undetected.

Rename handling:

- Re-stamp codegen outputs after patching externs in place, otherwise
  every patched module was needlessly recompiled on the next build.
- Patch instance chain ids as well as source spans, otherwise a moved
  file's old path lingered in externs and incremental output diverged
  from a clean build.
- Rebuild a moved module when corefn, docs or source-map targets are
  requested, since those embed the module's own source path.

This also adds a --no-cutoff flag to `purs compile` to rebuild everything
downstream of a change, matching older compilers, to help diagnose an
unexpected build result.

It also restores the cut-off test cases dropped in an earlier refactor,
ports the regression tests from the purserl fork, and adds some more
adversarial and rename and --no-cutoff tests in TestMake.hs