Fix non-deterministic parser printing

In _very_ rare cases (I had about 7 cases out of 32200 files!),
the order of how inherit-from bindings are printed when using
`nix-instantiate --parse` gets messed up.

The cause of this seems to be because the std::map the bindings are
placed in is keyed on a _pointer_, which then uses an
[implementation-defined strict total order](https://en.cppreference.com/w/cpp/language/operator_comparison#Pointer_total_order).

The fix here is to key the bindings on their displacement instead,
which maintains the same order as they appear in the file.

Unfortunately I wasn't able to make a reproducible test for this in the
source, there's something about the local environment that makes it
unreproducible for me.

However I was able to make a reproducible test in a Nix build on a Nix
version from a very recent master:

    nix build github:infinisil/non-det-nix-parsing-repro

src/libexpr/nixexpr.cc

@@ -82,7 +82,7 @@ void ExprAttrs::showBindings(const SymbolTable & symbols, std::ostream & str) co
         return sa < sb;
     });
     std::vector<Symbol> inherits;
-    std::map<ExprInheritFrom *, std::vector<Symbol>> inheritsFrom;
+    std::map<Displacement, std::vector<Symbol>> inheritsFrom;
     for (auto & i : sorted) {
         switch (i->second.kind) {
         case AttrDef::Kind::Plain:
@@ -93,7 +93,7 @@ void ExprAttrs::showBindings(const SymbolTable & symbols, std::ostream & str) co
         case AttrDef::Kind::InheritedFrom: {
             auto & select = dynamic_cast<ExprSelect &>(*i->second.e);
             auto & from = dynamic_cast<ExprInheritFrom &>(*select.e);
-            inheritsFrom[&from].push_back(i->first);
+            inheritsFrom[from.displ].push_back(i->first);
             break;
         }
         }
@@ -105,7 +105,7 @@ void ExprAttrs::showBindings(const SymbolTable & symbols, std::ostream & str) co
     }
     for (const auto & [from, syms] : inheritsFrom) {
         str << "inherit (";
-        (*inheritFromExprs)[from->displ]->show(symbols, str);
+        (*inheritFromExprs)[from]->show(symbols, str);
         str << ")";
         for (auto sym : syms) str << " " << symbols[sym];
         str << "; ";