tree-wide: fix a pile of lints

This:
- Converts a bunch of C style casts into C++ casts.
- Removes some very silly pointer subtraction code (which is no more or
  less busted on i686 than it began)
- Fixes some "technically UB" that never had to be UB in the first
  place.
- Makes finally follow the noexcept status of the inner function. Maybe
  in the future we should ban the function from not being noexcept, but
  that is not today.
- Makes various locally-used exceptions inherit from std::exception.

Change-Id: I22e66972602604989b5e494fd940b93e0e6e9297

src/libstore/binary-cache-store.cc

@@ -3,17 +3,15 @@
 #include "compression.hh"
 #include "derivations.hh"
 #include "fs-accessor.hh"
-#include "globals.hh"
 #include "nar-info.hh"
 #include "sync.hh"
 #include "remote-fs-accessor.hh"
-#include "nar-info-disk-cache.hh"
+#include "nar-info-disk-cache.hh" // IWYU pragma: keep
 #include "nar-accessor.hh"
 #include "thread-pool.hh"
 #include "signals.hh"
 
 #include <chrono>
-#include <future>
 #include <regex>
 #include <fstream>
 #include <sstream>
@@ -480,7 +478,8 @@ void BinaryCacheStore::addSignatures(const StorePath & storePath, const StringSe
        when addSignatures() is called sequentially on a path, because
        S3 might return an outdated cached version. */
 
-    auto narInfo = make_ref<NarInfo>((NarInfo &) *queryPathInfo(storePath));
+    // downcast: BinaryCacheStore always returns NarInfo from queryPathInfoUncached, making it sound
+    auto narInfo = make_ref<NarInfo>(dynamic_cast<NarInfo const &>(*queryPathInfo(storePath)));
 
     narInfo->sigs.insert(sigs.begin(), sigs.end());