fix(3p/nix): Compatibility with updated SymbolTable

The functions in SymbolTable have been renamed to match the Google
Style guide, and some debug-only functions have been removed.

third_party/nix/src/libexpr/lexer.l

@@ -72,7 +72,7 @@ static Expr * unescapeStr(SymbolTable & symbols, const char * s, size_t length)
         }
         else t += c;
     }
-    return new ExprString(symbols.create(t));
+    return new ExprString(symbols.Create(t));
 }