Define and use 'istring' type for some return types.

* (getTypeMap): Add mapping for 'istring'.
* kawa/istrings.scm: Use 'istring' for appropriate return types.
1 job for master in 5 minutes and 33 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build And Test
failed #468584657


Name Stage Failure
build_script Build And Test
  -e '(define out (open-output-file "tmpP"))' \
-e '(pprint-file "../gnu/kawa/slib/ppfile.scm" out)' \
-e '(close-output-port out)'
#tdiff tmpP ./ppfile.out
make[1]: diff: Command not found
make[1]: *** [Makefile:691: check-ppfile] Error 127
make[1]: Leaving directory '/builds/kashell/Kawa/testsuite'
make: *** [Makefile:1443: check-recursive] Error 1
ERROR: Job failed: exit code 1