treewide: fix a bunch of lints

Fixes:
- Identifiers starting with _ are prohibited
- Some driveby header dependency cleaning which wound up with doing some
  extra fixups.
- Fucking C style casts, man. C++ made these 1000% worse by letting you
  also do memory corruption with them with references.
  - Remove casts to Expr * where ExprBlackHole is an incomplete type by
    introducing an explicitly-cast eBlackHoleAddr as Expr *.
  - An incredibly illegal cast of the text bytes of the StorePath hash
    into a size_t directly. You can't DO THAT.

    Replaced with actually parsing the hash so we get 100% of the bits
    being entropy, then memcpying the start of the hash. If this shows
    up in a profile we should just make the hash parser faster with a
    lookup table or something sensible like that.
  - This horrendous bit of UB which I thankfully slapped a deprecation
    warning on, built, and it didn't trigger anywhere so it was dead
    code and I just deleted it. But holy crap you *cannot* do that.

    inline void mkString(const Symbol & s)
    {
        mkString(((const std::string &) s).c_str());
    }
- Some wrong lints. Lots of wrong macro lints, one wrong
  suspicious-sizeof lint triggered by the template being instantiated
  with only pointers, but the calculation being correct for both
  pointers and not-pointers.
- Exceptions in destructors strike again. I tried to catch the
  exceptions that might actually happen rather than all the exceptions
  imaginable. We can let the runtime hard-kill it on other exceptions
  imo.

Change-Id: I71761620846cba64d66ee7ca231b20c061e69710

src/libstore/path.cc

@@ -112,3 +112,13 @@ PathSet Store::printStorePathSet(const StorePathSet & paths) const
 }
 
 }
+
+std::size_t std::hash<nix::StorePath>::operator()(const nix::StorePath & path) const noexcept
+{
+    // It's already a cryptographic hash of 160 bits (assuming that nobody gives us bogus ones...), so just parse it.
+    auto h = nix::Hash::parseNonSRIUnprefixed(path.hashPart(), nix::HashType::SHA1);
+    // This need not be stable across machines, so bit casting the start of it is fine.
+    size_t r;
+    memcpy(&r, h.hash, sizeof(r));
+    return r;
+}