keep closure signatures apart when mangled

The argument list was flattened with one separator throughout, so
`((long long))' and `((long) (long))' named one struct. A capture
borrowing a name looked only at the local scope chain.

Makefile

@@ -98,7 +98,8 @@ sextest:
 
 SEX_TEST_PROGRAMS = hello-world lists comments unicode serialize features \
                     feature-flags lambdas compound-literals closures fixpoint \
-                    wildcards inference type-shapes unnamed-params
+                    wildcards inference type-shapes unnamed-params \
+                    closure-signatures
 
 # Multi-module linking is checked end to end; see tests/modules/Makefile.
 check-modules: sexc