procedures in "drop" contexts can return unspecified values

procedures in "drop" contexts can return unspecified values

* module/language/tree-il/compile-glil.scm (flatten): For applications in
  "drop" context, allow the procedure to return unspecified values
  (including 0 values).

* test-suite/tests/tree-il.test ("application"): Adapt test.

* module/srfi/srfi-18.scm (wrap): Clarify.

* test-suite/tests/srfi-18.test: Fix so that the expression importing
  srfi-18 is expanded before the tests. However the tests are still
  failing, something about 0-valued returns...