Don't assume the type of string::size_type

The code accidentally conflated `std::string::size_type` and `long unsigned int`.
This was fine on 64bits machines where they are apparently the same in
practice, but not on 32bits. Fix that by using `std::string::size_type`
everywhere.

src/libutil/references.hh

@@ -26,7 +26,7 @@ public:
 struct RewritingSink : Sink
 {
     const StringMap rewrites;
-    long unsigned int maxRewriteSize;
+    std::string::size_type maxRewriteSize;
     std::string prev;
     Sink & nextSink;
     uint64_t pos = 0;