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/derived-path.hh

@@ -78,10 +78,12 @@ struct SingleDerivedPathBuilt {
     DECLARE_CMP(SingleDerivedPathBuilt);
 };
 
-using _SingleDerivedPathRaw = std::variant<
+namespace derived_path::detail {
+using SingleDerivedPathRaw = std::variant<
     DerivedPathOpaque,
     SingleDerivedPathBuilt
 >;
+}
 
 /**
  * A "derived path" is a very simple sort of expression (not a Nix
@@ -94,8 +96,8 @@ using _SingleDerivedPathRaw = std::variant<
  * - built, in which case it is a pair of a derivation path and an
  *   output name.
  */
-struct SingleDerivedPath : _SingleDerivedPathRaw {
-    using Raw = _SingleDerivedPathRaw;
+struct SingleDerivedPath : derived_path::detail::SingleDerivedPathRaw {
+    using Raw = derived_path::detail::SingleDerivedPathRaw;
     using Raw::Raw;
 
     using Opaque = DerivedPathOpaque;
@@ -201,10 +203,12 @@ struct DerivedPathBuilt {
     DECLARE_CMP(DerivedPathBuilt);
 };
 
-using _DerivedPathRaw = std::variant<
+namespace derived_path::detail {
+using DerivedPathRaw = std::variant<
     DerivedPathOpaque,
     DerivedPathBuilt
 >;
+}
 
 /**
  * A "derived path" is a very simple sort of expression that evaluates
@@ -216,8 +220,8 @@ using _DerivedPathRaw = std::variant<
  * - built, in which case it is a pair of a derivation path and some
  *   output names.
  */
-struct DerivedPath : _DerivedPathRaw {
-    using Raw = _DerivedPathRaw;
+struct DerivedPath : derived_path::detail::DerivedPathRaw {
+    using Raw = derived_path::detail::DerivedPathRaw;
     using Raw::Raw;
 
     using Opaque = DerivedPathOpaque;