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/libcmd/built-path.hh

@@ -20,13 +20,15 @@ struct SingleBuiltPathBuilt {
     DECLARE_CMP(SingleBuiltPathBuilt);
 };
 
-using _SingleBuiltPathRaw = std::variant<
+namespace built_path::detail {
+using SingleBuiltPathRaw = std::variant<
     DerivedPathOpaque,
     SingleBuiltPathBuilt
 >;
+}
 
-struct SingleBuiltPath : _SingleBuiltPathRaw {
-    using Raw = _SingleBuiltPathRaw;
+struct SingleBuiltPath : built_path::detail::SingleBuiltPathRaw {
+    using Raw = built_path::detail::SingleBuiltPathRaw;
     using Raw::Raw;
 
     using Opaque = DerivedPathOpaque;
@@ -65,17 +67,19 @@ struct BuiltPathBuilt {
     DECLARE_CMP(BuiltPathBuilt);
 };
 
-using _BuiltPathRaw = std::variant<
+namespace built_path::detail {
+using BuiltPathRaw = std::variant<
     DerivedPath::Opaque,
     BuiltPathBuilt
 >;
+}
 
 /**
  * A built path. Similar to a DerivedPath, but enriched with the corresponding
  * output path(s).
  */
-struct BuiltPath : _BuiltPathRaw {
-    using Raw = _BuiltPathRaw;
+struct BuiltPath : built_path::detail::BuiltPathRaw {
+    using Raw = built_path::detail::BuiltPathRaw;
     using Raw::Raw;
 
     using Opaque = DerivedPathOpaque;