Add debug string and ODesc support to HashKey class

This allows tracing of hash key buffer reservations, reads, and writes via a new
debug stream, and supports printing a summary of a HashKey object via
Describe(). The latter comes in handy e.g. in TableVal::Describe() (where
including the hash key is now available but commented out).

src/Val.cc

@@ -2395,6 +2395,9 @@ void TableVal::Describe(ODesc* d) const
 			d_ptr->SP();
 			}
 
+		// The following shows the HashKey state as well:
+		// k->Describe(d_ptr);
+		// d_ptr->SP();
 		vl->Describe(d_ptr);
 
 		if ( table_type->IsSet() )