Make the Store API more type-safe

Most functions now take a StorePath argument rather than a Path (which
is just an alias for std::string). The StorePath constructor ensures
that the path is syntactically correct (i.e. it looks like
<store-dir>/<base32-hash>-<name>). Similarly, functions like
buildPaths() now take a StorePathWithOutputs, rather than abusing Path
by adding a '!<outputs>' suffix.

Note that the StorePath type is implemented in Rust. This involves
some hackery to allow Rust values to be used directly in C++, via a
helper type whose destructor calls the Rust type's drop()
function. The main issue is the dynamic nature of C++ move semantics:
after we have moved a Rust value, we should not call the drop function
on the original value. So when we move a value, we set the original
value to bitwise zero, and the destructor only calls drop() if the
value is not bitwise zero. This should be sufficient for most types.

Also lots of minor cleanups to the C++ API to make it more modern
(e.g. using std::optional and std::string_view in some places).

src/libstore/s3-binary-cache-store.cc

@@ -222,7 +222,7 @@ struct S3BinaryCacheStoreImpl : public S3BinaryCacheStore
        fetches the .narinfo file, rather than first checking for its
        existence via a HEAD request. Since .narinfos are small, doing
        a GET is unlikely to be slower than HEAD. */
-    bool isValidPathUncached(const Path & storePath) override
+    bool isValidPathUncached(const StorePath & storePath) override
     {
         try {
             queryPathInfo(storePath);
@@ -382,9 +382,9 @@ struct S3BinaryCacheStoreImpl : public S3BinaryCacheStore
             throw NoSuchBinaryCacheFile("file '%s' does not exist in binary cache '%s'", path, getUri());
     }
 
-    PathSet queryAllValidPaths() override
+    StorePathSet queryAllValidPaths() override
     {
-        PathSet paths;
+        StorePathSet paths;
         std::string marker;
 
         do {
@@ -405,7 +405,7 @@ struct S3BinaryCacheStoreImpl : public S3BinaryCacheStore
             for (auto object : contents) {
                 auto & key = object.GetKey();
                 if (key.size() != 40 || !hasSuffix(key, ".narinfo")) continue;
-                paths.insert(storeDir + "/" + key.substr(0, key.size() - 8));
+                paths.insert(parseStorePath(storeDir + "/" + key.substr(0, key.size() - 8) + "-unknown"));
             }
 
             marker = res.GetNextMarker();