Allow external code using libnixexpr to add types

Code that links to libnixexpr (e.g. plugins loaded with importNative, or
nix-exec) may want to provide custom value types and operations on
values of those types. For example, nix-exec is currently using sets
where a custom IO value type would be more appropriate. This commit
provides a generic hook for such types in the form of tExternal and the
ExternalBase virtual class, which contains all functions necessary for
libnixexpr's type-polymorphic functions (e.g. `showType`) to be
implemented.

src/libexpr/value-to-json.cc

@@ -80,10 +80,21 @@ void printValueAsJSON(EvalState & state, bool strict,
             break;
         }
 
+	case tExternal:
+            v.external->printValueAsJSON(state, strict, str, context);
+            break;
+
         default:
             throw TypeError(format("cannot convert %1% to JSON") % showType(v));
     }
 }
 
 
+void ExternalValueBase::printValueAsJSON(EvalState & state, bool strict,
+      std::ostream & str, PathSet & context)
+{
+    throw TypeError(format("cannot convert %1% to JSON") % showType());
+}
+
+
 }