hashlib: only include in one place

kernel/hashlib.h

@@ -34,6 +34,8 @@ namespace hashlib {
  * Hashlib doesn't know how to hash silly Yosys-specific types.
  * Hashlib doesn't depend on Yosys and can be used standalone.
  * Please don't use hashlib standalone for new projects.
+ * Never directly include kernel/hashlib.h in Yosys code.
+ * Instead include kernel/yosys_common.h
  *
  * The hash_ops type is now always left to its default value, derived
  * from templated functions through SFINAE. Providing custom ops is

kernel/yosys_common.h

@@ -134,8 +134,7 @@ YOSYS_NAMESPACE_BEGIN
 // Note: All headers included in hashlib.h must be included
 // outside of YOSYS_NAMESPACE before this or bad things will happen.
 #ifdef HASHLIB_H
-#  undef HASHLIB_H
-#  include "kernel/hashlib.h"
+#  error You've probably included hashlib.h under two namespace paths. Bad idea.
 #else
 #  include "kernel/hashlib.h"
 #  undef HASHLIB_H