Handle circular lists in display procedure.

Fixes GitLab issue #78 "Cycles detection in REPL".
* DisplayFormat.java (schemeDisplayFormat): Set checkSharing.
(schemeDisplaySimpleFormat): New static.
* circlist.scm: Test display of circular list.
1 job for master in 5 minutes and 11 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build And Test
failed #849956960


Name Stage Failure
build_script Build And Test
  -e '(pprint-file "../gnu/kawa/slib/ppfile.scm" out)' \
-e '(close-output-port out)'
#tdiff tmpP ./ppfile.out
make[1]: diff: No such file or directory
make[1]: *** [Makefile:691: check-ppfile] Error 127
make[1]: Leaving directory '/builds/kashell/Kawa/testsuite'
make: *** [Makefile:1443: check-recursive] Error 1
Cleaning up file based variables
ERROR: Job failed: exit code 1