Reduce substitution memory consumption

copyStorePath() now pipes the output of srcStore->narFromPath()
directly into dstStore->addToStore(). The sink used by the former is
converted into a source usable by the latter using
boost::coroutine2. This is based on [1].

This reduces the maximum resident size of

  $ nix build --store ~/my-nix/ /nix/store/b0zlxla7dmy1iwc3g459rjznx59797xy-binutils-2.28.1 --substituters file:///tmp/binary-cache-xz/ --no-require-sigs

from 418592 KiB to 53416 KiB. (The previous commit also reduced the
runtime from ~4.2s to ~3.4s, not sure why.) A further improvement will
be to download files into a Sink.

[1] https://github.com/NixOS/nix/compare/master...Mathnerd314:dump-fix-coroutine#diff-dcbcac55a634031f9cc73707da6e4b18

Issue #1969.

src/libutil/serialise.hh

@@ -61,6 +61,8 @@ struct Source
     virtual size_t read(unsigned char * data, size_t len) = 0;
 
     virtual bool good() { return true; }
+
+    std::string drain();
 };
 
 
@@ -191,6 +193,27 @@ struct LambdaSink : Sink
 };
 
 
+/* Convert a function into a source. */
+struct LambdaSource : Source
+{
+    typedef std::function<size_t(unsigned char *, size_t)> lambda_t;
+
+    lambda_t lambda;
+
+    LambdaSource(const lambda_t & lambda) : lambda(lambda) { }
+
+    size_t read(unsigned char * data, size_t len) override
+    {
+        return lambda(data, len);
+    }
+};
+
+
+/* Convert a function that feeds data into a Sink into a Source. The
+   Source executes the function as a coroutine. */
+std::unique_ptr<Source> sinkToSource(std::function<void(Sink &)> fun);
+
+
 void writePadding(size_t len, Sink & sink);
 void writeString(const unsigned char * buf, size_t len, Sink & sink);