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/libexpr/eval-inline.hh

@@ -31,7 +31,7 @@ Value * EvalState::allocValue()
 #endif
 
     nrValues++;
-    return (Value *) p;
+    return static_cast<Value *>(p);
 }
 
 
@@ -54,10 +54,10 @@ Env & EvalState::allocEnv(size_t size)
         void * p = *env1AllocCache;
         *env1AllocCache = GC_NEXT(p);
         GC_NEXT(p) = nullptr;
-        env = (Env *) p;
+        env = static_cast<Env *>(p);
     } else
 #endif
-        env = (Env *) gcAllocBytes(sizeof(Env) + size * sizeof(Value *));
+        env = static_cast<Env *>(gcAllocBytes(sizeof(Env) + size * sizeof(Value *)));
 
     /* We assume that env->values has been cleared by the allocator; maybeThunk() and lookupVar fromWith expect this. */