Make RewritingSink accept a map of rewrites

Giving it the same semantics as `rewriteStrings`.
Also add some tests for it

src/libutil/references.hh

@@ -0,0 +1,56 @@
+#pragma once
+///@file
+
+#include "hash.hh"
+
+namespace nix {
+
+class RefScanSink : public Sink
+{
+    StringSet hashes;
+    StringSet seen;
+
+    std::string tail;
+
+public:
+
+    RefScanSink(StringSet && hashes) : hashes(hashes)
+    { }
+
+    StringSet & getResult()
+    { return seen; }
+
+    void operator () (std::string_view data) override;
+};
+
+struct RewritingSink : Sink
+{
+    const StringMap rewrites;
+    long unsigned int maxRewriteSize;
+    std::string prev;
+    Sink & nextSink;
+    uint64_t pos = 0;
+
+    std::vector<uint64_t> matches;
+
+    RewritingSink(const std::string & from, const std::string & to, Sink & nextSink);
+    RewritingSink(const StringMap & rewrites, Sink & nextSink);
+
+    void operator () (std::string_view data) override;
+
+    void flush();
+};
+
+struct HashModuloSink : AbstractHashSink
+{
+    HashSink hashSink;
+    RewritingSink rewritingSink;
+
+    HashModuloSink(HashType ht, const std::string & modulus);
+
+    void operator () (std::string_view data) override;
+
+    HashResult finish() override;
+};
+
+}