psyntax's labels and marks now unique over a read/write boundary

psyntax's labels and marks now unique over a read/write boundary

* module/ice-9/psyntax.scm (gen-label, new-mark): Labels and marks need
  to be unique across read/write boundaries -- it's not sufficient for
  them to simply be unique within a process by virtue of (string #\i)
  constructing a new object. This used to mostly work before, because
  the collapsing of duplicate constants didn't catch many syntax-object
  cases -- but for some reason the attached test case brings out the
  problem. So switch to use gensyms. Potentially more costly, but it's
  what upstream psyntax does now.

  This bug took me fully two days to figure out.

* module/ice-9/psyntax-pp.scm: Regenerated.

* test-suite/tests/syncase.test ("serializable labels and marks"): Add
  test case.