The vast majority of changes was pretty straight-forward: we need an
AsyncIoRoot per thread to resolve promises we get from Lix. The `aio`
object is created per thread and passed as lvalue ref instead of making
it a field of the State struct since that struct is shared between the
threads.

The biggest change affects the NAR extractor: parseAndCopyDump has been
removed, instead the `nar::parse` and `nar::dump` parts are public now.
This won't work with the original parse visitor anymore, so instead the
NAR is parsed once and dumped again to copy it to the binary cache. In
between, the generator is walked through to search for relevant files
(such as build products) and after that the generators are recreated for
`AsyncGeneratorInputStream` to read again.

@@ -397,6 +399,7 @@ void State::abortUnsupported()
             bool stepFinished = false;
 
             failStep(
+                    aio,

@@ -41,1 +38,3 @@
-    std::unordered_set<Path> filesToKeep {
+static Generator<nar::Entry> discoverHydraDataInNAR(
+    NarMemberDatas &members,
+    const Path &&prefix,

@@ -47,2 +50,2 @@
-    NarMemberDatas & members;
-    Path prefix;
+    nix::overloaded handlers {
+        [&](nar::File f) -> Generator<nar::Entry> {

@@ -75,0 +94,4 @@
+                            std::cref(e->first),
+                            *discoverHydraDataInNAR(
+                                members,
+                                std::move(prefix),

@@ -96,0 +127,4 @@
+        auto items = nar::parse(source);
+        co_yield nar::dump(*discoverHydraDataInNAR(
+            members,
+            std::move(prefix),

@@ -96,0 +129,4 @@
+            members,
+            std::move(prefix),
+            "",
+            std::move(*items.next())

@@ -96,0 +130,4 @@
+            std::move(prefix),
+            "",
+            std::move(*items.next())
+        ).next());

@@ -49,0 +75,4 @@
+            members.insert_or_assign(
+                prefix + pathSuffix,
+                NarMemberData{.type = FSAccessor::Type::tSymlink});
+            return std::move(sl);