Reverted the recent hash.h changes. Instead use -Wno-duplicate-decl-specifier with clang. The modified version required hash table users to know the structs' contents, which isn't otherwise necessary.

Fixed "duplicate const" warnings with new clang.

Reverted some of the last hash table changes. They caused hash table operations to dereference pointers, which caused a compiler error if the type wasn't known.

Simplified hash table union now that it again works only with pointers.

Added missing hash-decl.h