read a var's type as a type, not as a call

`walk-parts' walked the whole `var' form, and a `fn' type's parameter
list is shaped like a call: `(fn ((c int)) int)' came out as
`int (*fp)(c int)'. A closure literal had no type, so calling one in
place missed the rewrite; sextest fed sexc symbols Sex cannot read.

Makefile

@@ -108,6 +108,7 @@ SEX_TEST_PROGRAMS = c99 \
                     inference \
                     lambdas \
                     lists \
+                    operators \
                     serialize \
                     type-shapes \
                     unicode \