util/DereferenceIterator, TerminatedArray: added operator-(const IteratorType&) to DereferenceIterator and TerminatedArray::iterator

It is required by std::distance and some std algos.

src/util/DereferenceIterator.hxx

@@ -82,6 +82,10 @@ public:
 		return DereferenceIterator{original - n};
 	}
 
+	constexpr auto operator-(const DereferenceIterator<IT, VT>& other) const noexcept {
+		return std::distance(other.original, original);
+	}
+
 	/* this is a template to allow comparisons with sentinel end
 	   iterators */
 	template<typename IT2>

src/util/TerminatedArray.hxx

@@ -101,6 +101,10 @@ public:
 			return iterator{cursor - n};
 		}
 
+		constexpr auto operator-(const iterator& other) const noexcept {
+			return std::distance(other.cursor, cursor);
+		}
+
 		reference operator*() const noexcept {
 			return *cursor;
 		}