Fix RSET_IS_MEMBER macro parameter mismatch

The RSET_IS_MEMBER macro had a parameter named 'sobj' but the macro
body used 'set' instead, causing the first argument to be ignored.

This worked by accident because all current callers use a variable
named 'set', but would cause compilation failure if called with a
differently named variable:

    error: use of undeclared identifier 'set'

Changed the parameter name from 'sobj' to 'set' to match the macro
body and be consistent with other RSET_* macros.

Author: afurm

set.c

@@ -116,7 +116,7 @@ static ID id_class_methods;
 #define RSET_SIZE(set) set_table_size(RSET_TABLE(set))
 #define RSET_EMPTY(set) (RSET_SIZE(set) == 0)
 #define RSET_SIZE_NUM(set) SIZET2NUM(RSET_SIZE(set))
-#define RSET_IS_MEMBER(sobj, item) set_table_lookup(RSET_TABLE(set), (st_data_t)(item))
+#define RSET_IS_MEMBER(set, item) set_table_lookup(RSET_TABLE(set), (st_data_t)(item))
 #define RSET_COMPARE_BY_IDENTITY(set) (RSET_TABLE(set)->type == &identhash)
 
 struct set_object {