ltlfilt: add a --save-part-file option

* bin/ltlfilt.cc: Add support for --save-part-file.
* NEWS, doc/org/ltlfilt.org: Mention it.
* tests/core/ltlfilt.test: Test it.

tests/core/ltlfilt.test

@@ -872,3 +872,19 @@ diff expected out
 # would stick to the rest of the file unless overridden.
 run 2 ltlfilt -Ffile/1 -Ffile
 :
+
+
+run 0 ltlfilt -f 'i1 U o2 U i3' --save-part=p.part
+cat > exp.part<<EOF
+.inputs i1 i3
+.outputs o2
+EOF
+diff exp.part p.part
+
+run 0 ltlfilt -f 'a U b U c' --ins=a --relabel=io --save-part >out
+cat > exp<<EOF
+.inputs i0
+.outputs o0 o1
+i0 U (o0 U o1)
+EOF
+diff exp out